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 has a premonition of some sort of danger that awaits him at Hogwarts. Maybe he dreams about something bad happening there and that makes him not want to go. It could also be that he has some family - related issues. He might feel that going to Hogwarts is going against the wishes of his aunt and uncle, who have raised him in a certain way.
One possible reason could be that Harry might feel he's not ready for marriage. He has been through so much trauma from fighting Voldemort and might need more time to focus on himself and his own healing. Another reason could be that he might view Ginny more as a sister or a close friend at this point in the fanfiction, rather than a romantic partner.
One possible reason could be that Harry has grown tired of being Dumbledore's pawn. He might feel that he has been constantly pushed into dangerous situations for Dumbledore's grand plans and he just wants to focus on his own life and safety for once.
One possible reason could be that he's tired of the constant pressure and spotlight that comes with competing. He has been through so much already and just wants a break from the high - stakes competitions like the Triwizard Tournament. Maybe he's also seen how competition can bring out the worst in people and doesn't want to be part of that.
One possible reason could be that he's tired of being used by the British wizarding world. He has faced so many challenges and been through a great deal of trauma, like fighting Voldemort multiple times. He might feel that it's time for others to take responsibility and that he wants to focus on his own life for a change.
One possible reason could be his fear of losing his independence. Harry has always been a bit of a lone wolf, used to relying on himself. Marriage might seem like a threat to his freedom to do as he pleases.
One possible reason could be that he wants to prove his own strength without relying on a wand. In the wizarding world, wands are seen as a crutch for magic, and Harry might want to show that he can be powerful even without one.
One possible reason could be that Harry is still haunted by his past traumas and feels he's not in the right mental state to marry anyone. His experiences with Voldemort and the losses he's endured might make him reluctant to bring Ginny into a potentially unstable future.
One possible reason could be a misunderstanding. Maybe Harry overheard Hermione saying something that he misinterpreted, like she was criticizing his actions but in reality she was just worried. For example, if Hermione was expressing concerns about Harry's risky adventures in a way that Harry thought was her doubting his abilities.
Maybe Harry has some unresolved issues from his past, like the trauma of losing his parents and many of his friends during the fight against Voldemort. These could make him hesitant to enter into a marriage with Ginny. He might also be worried about how the wizarding world would view their marriage, especially if there are still some who oppose him or his family.
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.