One possible event could be that Dumbledore fails to protect someone Harry loves dearly. Like if a close friend of Harry's was killed while under Dumbledore's supposed protection, Harry would hold Dumbledore responsible. Another event might be if Dumbledore lies to Harry about his family history. For example, if he hid some important information about Harry's parents that would change Harry's perception of them.
One possible event could be that Lupin lied to Harry about something important. In the world of Harry Potter, trust is a huge factor. If Lupin deceived Harry, it could easily turn Harry against him. Say, Lupin lied about his whereabouts during a crucial time when something bad happened, and Harry suspected Lupin's involvement.
It could be that Harry finds out about Dumbledore's secrets regarding the Deathly Hallows. If Harry thinks Dumbledore was more interested in the Hallows for his own purposes rather than for the greater good, it might make him hate Dumbledore.
It could be that in the fanfic, Team 7 starts to act in a way that goes against Naruto's moral code. For example, if they start using cruel or unethical methods to achieve their goals. Naruto, being the character he is, would not stand for such behavior and this could lead to his hatred. Additionally, if there is a personal loss that Naruto experiences which he blames on Team 7, that would also contribute to his negative feelings.
Perhaps in this fanfic, the Weasleys have some actions that conflict with Harry and Hermione's values. For example, if the Weasleys were to make a deal with a dark wizard for the sake of their family's safety and Harry and Hermione found out. This could lead to a rift and hatred. Also, it could be related to a love triangle situation in a very convoluted way. Maybe one of the Weasleys unknowingly gets in the way of Harry and Hermione's relationship, or vice versa, which causes animosity.
One possible reason could be a misunderstanding. Maybe Lupin did something that Harry misinterpreted as betrayal or negligence. For example, if Lupin was forced to keep a secret from Harry for his own safety, but Harry found out in a way that made him feel deceived.
One possible reason could be his lingering resentment towards James Potter, Harry's father. Snape and James had a long - standing feud during their school days. Since Harry looks a lot like James, Snape might be transferring his old grudges onto Harry.
The marriage could also have an impact on Harry's friends. They might be caught in the middle between their loyalty to Harry and their respect for Dumbledore. For example, Ron and Hermione might struggle to support Harry wholeheartedly if Dumbledore makes his disapproval clear. Moreover, the witch herself would likely face a great deal of hostility from Dumbledore's supporters, which could make her and Harry's life together quite difficult.
One key event could be a situation where Harry discovers a hidden magical ability that Dumbledore was unaware of. This new - found ability allows Harry to approach a problem in a different way and outwit Dumbledore. For example, if Harry discovers a way to communicate with magical creatures in a more profound way than Dumbledore ever thought possible.
Maybe Dumbledore discovers some dark secret about Harry's past that he can't handle. It could also be that Dumbledore is trying to protect Harry in a really strange way, by pushing him away from the dangerous wizarding world around Hogwarts.
In some fanfictions, Serena might have accidentally released one of Ash's Pokémon. Ash is very attached to his Pokémon, and such an act could make him angry and start to hate her.