One possible reason could be that he's under a great deal of stress. Maybe there are new threats to Berk that he's trying to deal with alone and it's getting to him. Another reason might be personal issues, like if he's having problems with his relationship with Astrid or his father. Or perhaps he's frustrated with the dragons not following his commands as he expects, which makes him lash out at everyone around him.
Another theme is growth. In many fanfics, both Hiccup and Ruffnut grow as individuals while also growing closer as a couple. For example, Hiccup might help Ruffnut see the value in strategy, and Ruffnut might help Hiccup be more bold and take risks. There are often adventures that they go through together which contribute to this growth. Also, the theme of acceptance is prevalent. They accept each other's flaws and quirks, which is a big part of their relationship development.
Astrid would definitely be a main character. She could be the one who helps Hiccup readjust to life in Berk after rehab. She might be a bit hesitant at first, but then realizes that Hiccup has truly changed for the better.
It can create a lot of tension. The other characters may start to avoid him, which can split the group. For example, if Toothless is also affected by Hiccup's mood, it can change their relationship dynamic. This can lead to new plotlines as the other characters try to figure out what's wrong with Hiccup and how to fix it.
In this fanfiction, after getting out of rehab, Hiccup might face some skepticism from the people of Berk. But he's a changed man. He starts by sharing his experiences in rehab, how he overcame his inner demons. This new - found openness could inspire the other Vikings, especially the younger ones who look up to him. He then gets back into dragon - riding, but with a more cautious and responsible approach, showing that he's truly grown.
One great httyd hiccup x ruffnut fanfic is 'A Different Bond' on FanFiction.net. It explores a unique relationship between Hiccup and Ruffnut that develops over time through their adventures.
One popular one could be 'The Lost Tales of Hiccup and Astrid'. It delves deeper into their relationship as adults, with new adventures in the Viking world.
There would be a shift in their dynamic as they prepare for a family. Hiccup, who is used to being adventurous, would have to focus more on home and stability. Astrid, while still strong - willed, would become more nurturing. They would need to find a new balance in their relationship, between their old selves and their new roles as parents - to - be.
Sure. 'The Maturity of Hiccup and Astrid' is a great one. It shows how Hiccup has grown from a boy into a wise leader and Astrid by his side, becoming a strong female figure. Their individual growth and how they support each other is really well - portrayed.
If Astrid gets injured, it could strengthen her relationship with Hiccup. Hiccup would be very worried about her and would do everything he can to take care of her. This would show Astrid how much he cares, and she would likely appreciate his efforts, bringing them closer together.