Perhaps Steve in the fanfiction was involved in a cover - up of some sort that affected Peter. Say, Steve knew about a threat coming towards Peter but didn't tell him in time because he thought Peter couldn't handle it. When Peter found out, he felt betrayed and thus couldn't forgive Steve. Also, if Steve accidentally caused harm to one of Peter's friends or allies while on a mission, that could be a major factor in Peter's unforgiveness.
Maybe Gryffindor did something that seriously violated Harry's principles. For example, he might have endangered Harry's friends or made a decision that put Harry in great danger without his consent.
Maybe Peter Parker has his own distinct view of Steve. He might feel that fanfiction often distorts Steve's real character or the relationships in the original story, so he doesn't like it.
It might start with a small incident where Steve dismisses Peter's ideas or concerns. This initial rift then grows as more events unfold. For example, Steve might take credit for something Peter did, which makes Peter angry and less likely to forgive.
One possible reason could be a major betrayal. Maybe Hermione made a decision that seriously endangered Harry's life or the lives of their friends, like revealing a crucial secret to the enemy. This would be a huge breach of trust and could lead to Harry not forgiving her.
It could also be due to a long - standing pattern of behavior. If Ron had been constantly critical or undermining of Harry in the fanfiction, Harry might reach a point where he just can't forgive. Say Ron always made fun of Harry's decisions in front of others, which gradually eroded Harry's trust and patience.
It could also be due to Sakura constantly doubting Naruto's abilities in the story. Naruto has always been looked down upon by many in the original series, and if in the fanfiction Sakura continuously expresses her lack of faith in him, it could lead to his unforgiveness. For example, she might have refused to believe in Naruto's new jutsu or his plans to save the village, which made Naruto feel really disappointed and unable to forgive her.
One possible reason could be a misunderstanding. Maybe Steve Rogers made a decision during a mission that Peter Parker thought put civilians in danger. So in a fit of anger, Peter punches him. For example, if Steve chose to sacrifice a small area to save a larger one but Peter had friends in that small area.
He might think it's not in line with the real Steve. Fanfiction can be quite far - fetched sometimes.
One reason could be a difference in ideology. Steve might have very traditional views and Peter's more modern or reckless approach could clash with that, leading to Steve's outburst.
His double life is also a big factor. Constantly having to balance being Peter Parker, a normal student, and Spider - Man, a superhero with great responsibilities, can be extremely taxing. He might feel like he can't be fully himself in either role, which can cause a sense of isolation and ultimately depression. In fanfiction, this can be shown through his inner monologues and his interactions with others.