One possible reason could be a difference in ideology. Maybe Harry starts to question Dumbledore's decisions regarding the war against Voldemort. For example, Dumbledore's secrecy about certain things might make Harry feel left out and manipulated. Another reason could be personal pride. Harry has grown into a powerful wizard and may not want to be under Dumbledore's constant guidance and control any longer. He might think he can handle things on his own better. Also, it could be due to the influence of other characters. If some new friends or allies have different views on Dumbledore, they could sway Harry's opinion.
One reason could be a misunderstanding. Maybe Harry discovers some actions of Dumbledore that seem wrong at first glance, like the way he's been handling secrets about Harry's past or Hogwarts' safety. For example, Dumbledore's decision to keep Harry in the dark about certain prophecies might make Harry feel betrayed and lead to a conflict.
The change in the storyline would be multi - faceted. Emotionally, Harry would be on a different journey. He would be dealing with feelings of betrayal (if he believes Dumbledore has wronged him) and determination to prove himself right. In terms of the plot, the search for Horcruxes might take a different turn. Harry might not follow Dumbledore's clues as closely. He could end up finding Horcruxes in different ways or even missing some if he doesn't have Dumbledore's wisdom. And the final confrontation with Voldemort would be more unpredictable. Harry's rebellion could lead to a more evenly matched battle as he might not have all the advantages that Dumbledore's guidance would have provided.
One possible reason could be that Harry breaks school rules in the fanfiction. For example, if he ventures into the Forbidden Forest without permission. Dumbledore is strict about the safety of students and such disobedience might lead to punishment.
Maybe Harry finds out that Dumbledore knew more about his family's past than he let on. For instance, if Dumbledore had information about some dark secrets in the Potter family but didn't tell Harry, it could make Harry furious and prompt him to yell. Also, if Dumbledore's actions led to the death or harm of someone Harry cared about, like Sirius, Harry would likely be very angry with him.
One possible reason could be a misunderstanding. Maybe Harry overheard something that made him think Dumbledore was planning something against him that he didn't understand. For example, if he heard Dumbledore discussing a dangerous mission for him without his full knowledge.
Well, it could be due to corruption within Konoha. If Naruto witnesses high - ranking officials abusing their power or making decisions that harm the innocent, he might decide to rebel. He's always been about protecting the weak and standing up for what's right. And if Konoha starts to go against those principles, Naruto would not stand idly by. It could also be that he's framed by someone in the village and then has no choice but to fight back against the false accusations and the system that allows such things to happen.
One reason could be secrecy. If Harry finds out that Dumbledore has been keeping major secrets from him all along, like about his family or his true role in the wizarding world, he might start to hate him. Another reason could be manipulation. If it seems that Dumbledore has been manipulating Harry's actions for his own gain, that would be a valid cause for hatred in fanfiction.
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.
One possible reason could be that Harry has discovered something about himself or his past that he thinks Dumbledore might not approve of or might try to stop him from exploring further. For example, if he found out some dark magic connection in his family history that he wants to understand on his own without Dumbledore interfering.
Maybe Harry discovers some things from Dumbledore's past that seem unethical to him. For example, Dumbledore's association with Grindelwald in his youth. Harry, who has a strong sense of right and wrong, might find it hard to trust someone with such a controversial past. And in the fanfiction, the author might explore how these revelations shake Harry's faith in Dumbledore.