Well, if Hogwarts shuts down in a fanfic, it would change everything. There would be no Quidditch matches at the school, which is a big part of the students' lives. The different houses would lose their sense of community within the school walls. Also, the magical creatures in the Hogwarts grounds might be left without proper care. The students would have to rely more on their own resources and the knowledge they already have. It would be a very different world from the normal Hogwarts - centric stories we are used to.
For Harry Potter himself, if Hogwarts shuts down, he would lose his second home. He has a strong connection to the school, and it's where he has grown and learned a lot. Hermione would be distraught as she loves learning at Hogwarts. She might try to find a way to preserve the knowledge. Ron would be sad to lose the social aspect and the food. Overall, the friendships that have been formed at Hogwarts would be strained as students are separated.
In the fanfic, Harry Potter could meet the new generation of students. They would be in awe of him. He might also encounter some of the ghosts, like Nearly Headless Nick, who would surely have some interesting stories to share since Harry left. And there's a good chance he'll meet the caretaker, who would probably give him a hard time about coming back and causing a stir. Moreover, he could cross paths with some of the older students who idolize him and want to learn from his experiences.
It might be significant as it shows the deep engagement of fans with the Hogwarts setting. By reading fanfic set in Hogwarts, they can further immerse themselves in the magical world, experiencing different scenarios, relationships, and adventures that are created by fellow fans.
Maybe he hates it because it often distorts the real story or characters he knows. Fanfics can take the story in wild directions that don't align with his own experiences.
Well, in some fanfics, the idea that Hogwarts is a horrible school might come from a different perspective. Maybe the author wants to explore the dark side of the school that wasn't fully shown in the original books. For example, the strict rules could be seen as overly oppressive to some characters in the fanfic.
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 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.
Harry might initially be shocked and confused. He's used to Hogwarts being his safe haven, so when it turns against him, he'll be at a loss. He may try to prove his innocence by finding evidence or confronting those who are spreading lies.