Another possibility is that Harry has a personal journey he needs to undertake alone. Maybe he discovers some new aspect of his magic or his family history that he can't fully explore with Hermione and Ron. He might need to go to a place or study with someone who the others don't trust or are not allowed to go with, so he has to leave the group to follow this new path.
One reason could be a sense of self - sacrifice. Harry might think that by leaving the golden trio, he can keep them out of danger. For example, if he believes he is being targeted by a powerful dark force and doesn't want his friends to get hurt because of him.
One possible reason could be a misunderstanding. Maybe Ginny misinterprets something Harry does and feels she can't stay in the relationship. Another reason might be that she feels overshadowed by Harry's fame and the pressure of being part of the Potter family. It could also be that she has some personal goals or dreams that she realizes she can't pursue while being with Harry and taking care of the kids.
In certain fanfiction scenarios, Harry could leave the wizarding world to explore other magical realms or non - magical places. He may feel that his growth and understanding of magic and life are restricted within the wizarding world he knows. Maybe he meets a group of people from outside the wizarding world who offer him a new perspective and a chance at a different kind of life.
One reason could be to escape the trauma of the war. They've been through so much in England, and leaving gives them a chance to heal in a new environment.
Maybe he's tired of the spotlight in England. In some fanfictions, he could be seeking a new start away from the prying eyes and the constant media attention that comes with being in England.
One possible reason could be to seek new opportunities. In fanfiction, the States might be depicted as a place full of exciting prospects in various fields like magic (if it's a Harry Potter - related fanfic) or just in general life. For example, he could be interested in exploring different magical cultures there.
One reason could be a misunderstanding. Maybe there was a miscommunication between Harry and his friends, and he felt so hurt or angry that he decided to leave. For example, if his friends wrongly accused him of something he didn't do.
Perhaps he leaves in search of a more peaceful life. The wizarding world, despite its magic, has been filled with turmoil for him. He could be seeking a place where he can simply be Harry, not 'The Boy Who Lived' or the hero who defeated Voldemort. He may also be on a quest to discover more about his family's past in the Muggle world, something that has been somewhat overlooked in his life in the wizarding world.
In some fanfictions, Harry might leave his family because of a prophecy or a special mission. He might feel that he has to go it alone in order to fulfill this prophecy or complete the mission successfully. His family might not understand or be able to support him in this, so he decides to leave. Also, there could be jealousy or resentment within the family that makes the environment unbearable for Harry.
In some fanfictions, it might be due to an alternate reality. For example, if there was a different prophecy or a different turn of events at the time Voldemort attacked. Harry could leave his parents to fulfill a new - found destiny in this altered world. Also, it could be because of a miscommunication. A magical misunderstanding might lead Harry to believe he has to leave his parents for their own safety.