One possible reason could be that he feels constantly in danger at Hogwarts. With all the dark magic threats and attacks that keep happening, he might think leaving would keep his friends and himself safer. For example, after the Chamber of Secrets incident, he might have considered leaving to protect others from Voldemort's influence.
Maybe he's tired of the school politics. There are always power struggles among the different houses and some teachers. He could be fed up with the unfair treatment from some, like Snape's constant sniping at him. So he wants to leave and get away from all that drama.
Another reason might be that he misses the Muggle world. He grew up there and has family there. He could be longing for a more normal life away from the magic and all the responsibilities that come with being the Boy Who Lived at Hogwarts.
He might leave Hogwarts in fanfic due to a sense of disillusionment. Maybe he discovers some dark secrets about the school or the wizarding world in general that make him question his place there. It could be something related to the way the school is run or a conspiracy among the teachers that he stumbles upon.
One possible reason could be that he's tired of the constant danger at Hogwarts. There are always threats like Voldemort's followers or dark creatures lurking around. Another reason might be that he feels pressured by the expectations placed on him as 'the Chosen One'. Everyone expects him to save the day all the time, and it's a huge burden.
It could also be that the fanfic might interfere with the way he wants his story to be remembered. His adventures at Hogwarts were real and important to him, and when fanfic changes the narrative, it's like his story is being disrespected. For example, if a fanfic portrays him as cowardly when he was actually brave, that would surely make him dislike it.
One possible reason could be misinformation spread by the Ministry of Magic. They might have spread false rumors about Harry being a threat, which influenced the Hogwarts community. For example, if the Daily Prophet printed false stories about Harry's supposed dark magic activities, it could turn some students and staff against him.
One possible reason could be that he was under the influence of a powerful dark magic or a curse that made him lose control. Another reason might be that he was tricked into believing Hogwarts was a threat to the wizarding world in some extreme way. Maybe a powerful enemy manipulated him to think that destroying Hogwarts was the only way to save everyone.
Well, it could completely disrupt the relationships. For instance, his friendship with Ron and Hermione might be strained as they try to convince him to stay. This is different from the original where they are all united in their Hogwarts experience.
Another aspect could be the never - ending battle against Voldemort. The fear, the stress, and the constant danger might have taken a toll on his mental health. He might think that dying would end this cycle of fear and uncertainty.
One reason could be to give him a stronger connection to Hogwarts. As the heir, he might have special privileges or knowledge about the school. Another reason is to add a new dimension to his character. It makes him more than just a student who defeated Voldemort.
One possible reason could be fear. Maybe he's heard some scary rumors about Hogwarts, like the Forbidden Forest being full of dangerous creatures. Another reason might be that he doesn't want to leave his normal life behind. He has friends in the Muggle world and he might be reluctant to give that up for a world full of magic that he doesn't fully understand yet.
Perhaps he gets a chance to attend a special magical school or training program that requires him to leave the Dursleys. It could be a place where he can develop his powers without the Dursleys' interference. Also, he may be running away from a threat that the Dursleys are unable or unwilling to protect him from.