There were quite a few things Astrid didn't like about Hiccup.
For starters, Astrid hated Hiccup's point of view. Absolutely despised it. Of course, she loved him to bits most days, but sometimes he would say something completely random and deep and philosophical and so un-Viking-like it reminded Astrid of how different they actually were. She liked fighting and spears and axes, and he liked drawing and riding dragons and inventing something that sounds utterly stupid at first, and then only when it was proudly put in front of her and the rest of the tribe did she realize how incredibly genius it actually was.
So yeah, they had different points of views. And no matter how much she loved him, Astrid strongly preferred hers.
She knew he would never change. That much was obvious. Astrid had tried time and time again to make him come training with her, asking him to spar with her, and every single time he had responded with, 'I've got other things to do.' It irritated Astrid to the brink of ripping her hair out. She had tried before, when they were younger, to persuade him to act more Viking-like. But he refused, and she had given up eventually. The boy had no hope. Yes, it doesn't matter much now he's the hero of the village, but sometimes she just wished he would man-up, at least a little!
And then there was the sarcasm, by Thor, the sarcasm! He was always stinging her with his attitude. Astrid knew Hiccup had a way with words, and it was probably his only un-Viking-like quality that Astrid admired, but sometimes his charisma made her want to slap him. Every time he did something stupid, (which was a lot) Astrid would insult him with what was definitely a knee-slap worthy comeback for her, before he says something witty and funny and clever that made everyone crack up laughing. Half the time, Astrid had the mind to give him a mighty smack across his freckled cheek and walk away.
Then Astrid hated his arrogance. That boy was sometimes so arrogant he could make Snotlout look under-confident and self-pitying. Hiccup had always had a little haughtiness, but after saving the tribe and getting to keep Toothless his head grew so big Astrid was surprised he could still walk without falling over. Then again, she wasn't one to talk, and there was that little period where Hiccup was, albeit reaching top of the class, uneasy and for once, unsure of himself. But that was because he was training a dragon, which was ridiculous, but it didn't count. So Astrid didn't like his arrogance. It was probably his least-redeeming quality.
She didn't like his policies, either. He thought in a different way, he acted in a different way, he walked and he talked in a different way, (although the walking thing is probably down to his fake leg). While he thought that the Vikings should try to ally themselves against the rival tribes and live in peace, Astrid was practically begging Stoic to take her on those battleships to fight. Hiccup's politics were simple; no fighting, everybody should live in peace. And Astrid couldn't disagree with him more.
He was an embarrassment, as well. Not only to his father and his tribe and his dragon for the gods' sake, but her. 99% of the time, he would say something during a war meeting about the fact that maybe a diplomatic talk could solve the problem, and that Astrid agreed with him all the way. A diplomatic talk? Was the boy actually serious? And then all eyes would settle on Astrid disbelievingly and she would have a fit, spitting and swinging fists and weapons until she had to be sent out. Hiccup was embarrassing. It was as simple as that.
But, despite this incredibly long list of flaws, Hiccup wasn't all bad. Yes, he was arrogant, and irritating, and he and Astrid couldn't be more different, but through it all, he did have a quality. And it was because of this, Astrid forced herself to push through all his faults.
She didn't mind it when he smiled. When he held her when things were tough and she was doubting herself, and reminded her off all the good things. He brought out the best of her. Despite her desire to be as Viking-like as possible, she found herself wanting to impress him. Maybe even more, to her horror. When around him, she acted her best, tried to make him proud. Which was strange, because all he did was embarrass her. But when she felt a sudden pang of envy because he was the hero and she wasn't, he wouldn't boast. He reminded her that she was amazing, that she was better, and he made her a better person. He brought out the best in her.
Then he was clever, an expert, a genius, so intelligent Astrid always found herself looking to him for advice. Most of the time, he used his intelligence to irritate her using snarky comebacks and superiority, but on occasion, he would talk to her, use his tactic and uplifting choice of words to make her feel beautiful about herself. Vikings shouldn't feel beautiful. But for some reason, he made her enjoy the feeling. So about her earlier point, him bringing out the best in her, most of the time he did so using his expertise, which made her feel special because expertise wasn't something Vikings were best known for.
But still, they weren't perfect. They fought a lot. After a day spent fighting a long battle against a rival tribe, they would head back to his home. She would feel refreshed and excited about the day's events, whereas Hiccup would feel tired and fed up and slightly annoyed that no-one listened to his 'diplomatic talk' option, despite him being the hero. And then when they sorted out their injuries and cleaned each other's wounds, the inevitable argument would spark up and break out into a full-blown fight. Hiccup would try to stick to verbal, but Astrid would try to sneak in some weapons here and there, but always failed.
But she didn't mind. In fact, she only continued arguing like she genuinely hated him because she always knew how it ended up. Eventually, she began to look forward to their arguments because of the conclusion, until the point where she started one on purpose. Hiccup, being the clever guy he is, caught on quickly and another argument was born about the fact that Astrid shouldn't start arguments on purpose. So her technique didn't last very long, but it still worked in some aspects. After Hiccup ended the argument like he always did, they wound up next to each other all bloodied and wounded, like they always did. It was moments like this that made Astrid look forward to the arguments, because without the bad stuff there wouldn't be any good.
Then sometimes, when he was gone an undercover quest to collect information about another tribe, Astrid would lie in his bed and worry about it. She knew that he was going to be perfectly fine with Toothless at his side, but part of her always doubted... what if something happened? What if Toothless and Hiccup got separated? Hiccup wasn't strong enough to protect himself without his dragon, let alone with a false leg. And then the days would stretch into weeks and the weeks would stretch into months, and Astrid began to lose sleep with worry for Hiccup. And then he would come back, slightly damaged, slightly taller, and slightly cockier. Astrid wanted to slap herself for ever worrying about him, because he always ended up alright.
But it was him that she missed. She wanted him, she needed him, and she needed those special moments that he reserved just for her. Vikings were known for being selfish, but this was a different kind of selfishness. She was being selflessly selfish. Astrid relied on him to keep her safe, to be there for her, and being Astrid, wasn't satisfied with anything else. But being a selfish Viking, she continued pretending to hate him so he would make up and they would wind up next to each other, and Hiccup gave up protesting and decided it wasn't a bad tactic after all.
So despite his flaws, Hiccup's quality of being there for her, helping her to bring out the best from herself, was the reason she stayed with him.
But one of his more prominent flaws was the fact he covered up his flawless self. Hiccup acted like a completely different person when they weren't alone, and Astrid hated it was the heritage Hiccup came from. He was the son of the chief. People never trusted him before and dreaded the day Stoic would have to give up his throne, but now Hiccup was the tribe's saviour, he had many pressures and responsibilities that got in the way of their relationship. Then again, if he hadn't saved the village like he had done, Astrid would never have the relationship in the first place, so she accepted the fact that life would never be perfect. And she was definitely not one to question authority, so she was not about to get in the way of the tribe's wishes just for herself.
But it was his genes. Because of the fact Stoic the Vast was his father, Hiccup was always expected to be someone he wasn't. Only Astrid and Toothless ever saw his true side. Astrid hated being jealous of a dragon, but she had suspicions that it may have been narrowed down to Toothless. He was still different, though. Hiccup was skinny despite the genes he held from his massive, beefy father. That's what she loved about him. He wasn't his father's son. He was himself, her Hiccup. But she still wanted Hiccup to take off his hero-act and his genes, so she could see the real hero that lied underneath. The thoughtful, clever, scrawny Hiccup that had disappeared. She wanted the tribe to disappear, so they could do things properly.
Back to his flaws. Talking about the good things was making Astrid feel sad, and she needed some more ranting to make her feel better.
She didn't like the way he smiled. Sometimes, his smile was warm, kind and tender, and Astrid wanted to melt into his arms and stay there forever. But she didn't like his smirk. The smirk he got every time she realized that he was right and that he always had been. She never admitted that she was wrong and he was right, but she knew he knew she knew and the thought always brought the smirk back to his face. He got it whenever he said a clever comeback his well, and Astrid wanted nothing more than to wipe it off his face with a well-deserved smack.
She didn't like the way he smiled. Not the one when he thought he was right, anyway.
But at the end of the day, despite her rough Viking exterior, she found it in herself to forgive him. Astrid didn't know how, as every time they fought she told herself she would last longer without forgiving him, but at the end of it Astrid gave up and came to the realization that Hiccup is not an easy person to stay mad at for long. She always forgave him, and could never not. No matter how hard she tried.
Sometimes, she pondered, their relationship was like an egg. Not a very eloquent way to put it, but she was a Viking and Viking's were not known for eloquence, so it was as good as it got for her. But it was like an egg in the way that although sometimes seem plain and boring and bad, they can always chew through it to get to the gold in the middle. Her forgiveness was the yolk in the middle, and every time he did something stupid she would chew through the white.
Astrid would always manage to forgive him.
Because she loved that one quality that shone through his flaws.
One night, when he was on a war mission that the females weren't allowed to go on, Astrid had a dream. Her mind was transported to the day she had found him, in Toothless's clearing, (which is what it is now called) and she was sat on the rock, sharpening her axe threateningly. It hurt to remember when she had treated him like that. She regretted every second of it now. She recalled hearing him say those words, 'I'm so... leaving. That's right. We're leaving.'
Then her subconscious mind was whisked away again, and she was on the dock. She was crying out, screaming, someone's name, Hiccup's name, to a boat in the distance. It was too far away, too far, and all she could see was a black silhouette against a blood-red sky. Then a wave had swallowed him up and he was gone, despite Astrid's agonised yells.
She had woken up in a cold sweat, screaming out his name into the dark of her room, pitch black now her single candle had gone out.
Then he had come back the next day, alive and mostly well with a few scratches here and there. To his surprise, Astrid had flung herself into his arms and clutched his tunic tightly. He was dumbfounded, as he had no idea what had brought this on, but Astrid had squeezed the life out of him while making him promise he would never leave her. He had confusedly promised, but Astrid never left his side for the rest of that day.
But at least one good thing came out of the dream. It had made her think of what she'd miss, of all the good qualities. All those times she had persuaded herself she hated him, despised him, and only when he was leaving did she desperately want him back.
It helped her come to a conclusion. Despite his faults, Astrid couldn't live without Hiccup. No matter what, she would always be able to forgive him, and sometimes even acted like she hated him just to get to that special moment of forgiveness they shared together in the aftermath. Despite the expectations Hiccup received from everyone, including her, and his annoying tendencies to drive her mad with his sarcastic comments, they could only ever take act like themselves around each other.
Hiccup was different. Hiccup was skinny. And they couldn't live without each other.