One possible reason could be that he felt he had learned enough magic to face his enemies on his own. He might have thought that the traditional Hogwarts education was too slow or not relevant to his specific needs, like dealing with Voldemort. Another reason could be some sort of conflict with the Hogwarts faculty. Maybe he didn't agree with their methods or decisions and decided he was better off without their rules.
Well, perhaps in the fanfiction, Harry has a vision of a different future for himself that doesn't involve traditional Hogwarts education. He might have seen a path where he can make a greater impact on the wizarding world without the constraints of school. It could also be that he's being influenced by a new character in the fanfiction, someone who convinces him that there are better ways to develop his magic and knowledge outside of Hogwarts. This new influence could be from a different magical culture or a rogue wizard with unique ideas.
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.
Maybe he leaves because of a conflict with the Hogwarts staff. If the new headmaster or some teachers start implementing rules that Harry believes are wrong or unfair, like restrictions on defensive magic classes, he might decide to leave in protest.
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.
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 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.
One reason could be that it doesn't stay true to the original lore. For example, if the fanfiction changes the fundamental rules of magic in Hogwarts that were established in the original series. Another reason might be that the characters' personalities are distorted. Harry Potter is known for his courage and self - sacrifice, and if the fanfiction turns him into a cowardly or self - centered character, it could be rejected.
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.
Maybe he feels that the rules at Hogwarts are too restrictive and are hindering his ability to truly learn and grow as a wizard. He might think that the traditional teaching methods are not suitable for him and his unique style of magic, so he rebels against the system.
One reason could be that he was under a dark spell cast by a new enemy in the fanfiction. Maybe some dark wizard or witch sent a spell his way before he boarded the train, and it took effect during the journey.