Fanfiction can sometimes be full of wild speculations and ideas that go against the established rules of the wizarding world. Harry Potter knows how things really work in his world, so he wouldn't blindly trust something that contradicts that knowledge.
Since fanfiction is created by a wide range of people with different interpretations, there's a high chance of misinformation. Harry Potter values the truth of his own story and the world he lives in, so he won't easily believe in fanfiction which could mislead about his friends, his adventures, and the magic system.
One reason could be jealousy. In some fanfictions, if Harry has become very famous and powerful, he might think the Weasleys have different motives for being around him, like using his fame for their own gain. So he starts to lose trust in them.
In some fanfictions, there might be a new magical element or prophecy that makes Harry question Hermione's motives. If the prophecy says something about Hermione being involved in a dark plot, even if it's false, Harry may start to have doubts about her.
Perhaps there were some secrets that Ron was keeping from Harry. In the wizarding world, secrets can be a big deal. If Ron had a secret related to his family or some other important matter and Harry found out about it in a way that made it seem like Ron was hiding something bad, it could lead to Harry not trusting him. Also, Ron's friendship with other characters might make Harry jealous or suspicious. If Ron was getting closer to someone Harry didn't trust, it could spill over and make Harry question Ron's loyalty to him.
It could also be due to a series of unlucky events. Suppose Hermione's well - intentioned plans keep backfiring in a way that makes Harry question her judgment. Like when she tries to solve a problem but ends up making it worse, Harry might start to lose trust.
It could also be due to some actions of McGonagall that seemed suspicious to Harry. Perhaps she made a decision regarding Hogwarts or Harry's safety that Harry didn't fully understand at the time. In the complex world of the wizarding school, decisions are often made with many factors in mind, but Harry might not be aware of all of them, leading to his distrust.
In fanfiction, when Harry doesn't trust Dumbledore, it often leads to a shift in the power balance. Harry might take more matters into his own hands, which could have both positive and negative outcomes. Positively, he could become more self - reliant and develop his own unique way of dealing with threats. Negatively, he could end up putting himself and others in more danger without Dumbledore's wisdom to guide him.
One reason could be to explore the darker side of human nature. Even though Harry is a hero in the original series, fanfiction writers might want to see how he would be affected if he was pushed to his limits. Another reason is to create a more complex and controversial storyline. It grabs readers' attention as it goes against the normal perception of Harry.
Some fanfiction writers may have a different vision for Harry Potter's character. They might see him as more of a diplomat or a healer within the wizarding world, rather than a fighter. So, they write stories that reflect this different view of the character, where he uses his wits and other skills rather than engaging in physical or magical combat.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.