Maybe he has a very traditional view on literature. He might prefer to stick to the original works and not engage with the fan - created versions. Fanfiction often takes the original story in different directions which he may not like.
One possible reason could be that fanfiction often takes creative liberties with the original story and characters. Harry Potter might be seen as a very personal creation of J.K. Rowling, and she (or in this case, the fictional Harry Potter) may not want it to be distorted in fan - made stories. Fanfiction can sometimes change the essence of the characters or the plot in ways that go against the original vision.
Well, it could be because fanfiction often goes against the original vision of the author. Harry Potter might not want to be part of something that could distort his story or the world he belongs to.
Maybe he has his own personal reasons. He could be afraid of failure, or perhaps he doesn't like the competitive environment. Another possibility is that he has some moral objections to the nature of the competition in the fanfic.
There could be ethical reasons. Fanfic sometimes involves using characters and settings created by others without proper authorization in a more complex way than just being inspired. Harry might not want to get involved in teaching something that has such potential legal and ethical gray areas.
Maybe he has his own reasons. He might be tired of the responsibilities that come with being a hero. In the fanfic, perhaps he has seen the dark side of heroism, like the constant danger and the loss of personal freedom. He could also be a complex character who doesn't conform to the typical heroic image.
Maybe in the fanfic, Harry has his own ideas and beliefs that conflict with Dumbledore's. He might think Dumbledore's plans are too risky or not in line with his own sense of justice.
Maybe he has his own principles. In some fanfics, Harry might have a different view of SHIELD's methods or goals. He could be more of a lone wolf type and doesn't want to be part of a large organization like SHIELD.
Maybe in the fanfic, Harry has some unresolved personal issues. He could be haunted by his past traumas, like the death of his parents and his battles with Voldemort, which make him feel that he's not in the right mental state to get married.
Perhaps they fear that fanfiction could deviate too much from the original themes and values of the Harry Potter series. The official story has a certain moral and ethical framework, and fanfiction might distort it.
Well, Naruto has his own journey and story arc in the original Naruto series. The 'cra fanfiction' might deviate too much from what he has experienced and learned. He has grown up with certain beliefs and goals, and if the fanfiction doesn't respect those, he'll refuse it. Additionally, he might be concerned about how such a fanfiction could influence the perception of his friends and the village he loves. If it portrays them in a negative or inaccurate light, he would surely stay away from it.