Perhaps he has seen fanfiction that contains inaccurate information about the Naruto universe, such as wrong jutsu descriptions or incorrect interpretations of the ninja clans. This would make him dislike it as he is an educator in the Naruto world and has a certain standard for knowledge about it.
It's also possible that he simply doesn't like the idea of others taking creative liberties with something he holds dear. The Naruto story was carefully crafted, and fanfiction can be seen as a form of 'tampering' with that. He might feel that the original creators' vision should be respected more and that fanfiction often oversteps those boundaries.
He may dislike it because he feels that it undermines the real struggles and achievements of the ninja. In fanfiction, things might be made too easy or unrealistic. He has fought tooth and nail for everything in his ninja life, and seeing it made into something that doesn't represent that in fanfiction would make him hate it.
One reason could be jealousy. If in the fanfic, Sakura gets closer to someone else Naruto cares about, like Sasuke in a different way than he expected, it might make him hate her. For example, if Sakura teams up with Sasuke against Naruto's wishes in the fan - created plot.
One possible reason could be jealousy. Maybe in the fanfiction, Jiraiya gives more attention to another character, and Naruto, being young and impulsive, starts to hate him for it.
One possible reason is a misunderstanding. If there are rumors or false memories planted in Naruto's mind, he might believe his mom did something unforgivable. For instance, if he's made to think she was involved in the death of someone he cared about.
In some fanfics, there could be a misunderstanding. Suppose Kushina did something that Naruto misinterpreted as harmful to him or his friends. This misinterpretation could lead to his hatred. For example, if Kushina was involved in a mission that Naruto thought put the village at risk, but in reality, she had no choice.
One possible reason is that the person may find the fan - made interpretations of the Pokemon world too far - fetched. For example, some fanfictions might give Pokemon abilities that are completely against the established rules in the official games or anime. This can be off - putting to those who like the consistency of the official lore.
One possible reason is that the fanfiction distorts the original relationship between Bella and Sam in a way she finds unacceptable. For instance, if it creates a false romantic tension where there shouldn't be one.
It could also be that in the fanfiction, Percy is given a new power or ability that Annabeth sees as a threat. Or perhaps Percy is made to side with someone Annabeth dislikes, causing her to 'hate' him in that fan - created story. This shows how different plot elements in fanfiction can change the dynamics between characters.
It might be that in the fanfiction, Calypso represents something that Percy has come to dislike. Maybe she is associated with a force or an idea that Percy opposes, like a form of confinement if she tries to keep him on the island in a more extreme way than in the original tales.
One reason could be that she doesn't like how others are taking creative liberties with their relationship in the fanfiction. It's like someone else is trying to rewrite a story that she feels is special to her in a way that she doesn't approve of. Another possibility is that she simply doesn't like the idea of fictionalized versions of their relationship floating around, as it might seem disrespectful to her real - life experiences with Ash.