It might be due to a self - sacrificing decision on Harry's part. He could believe that by leaving, he would protect the Weasleys from the dark forces that are constantly after him. Since he has so many enemies, he might think that as long as he is around the Weasleys, they are in danger.
Another reason could be related to his identity. Harry has always felt a bit like an outsider, even within the Weasley family which has been so kind to him. He might feel that he needs to go out on his own to truly discover who he is, away from the family environment that has both sheltered and sometimes overshadowed him.
One possible reason could be a misunderstanding with his friends in Gryffindor. Maybe there was a big argument over something like a Quidditch match strategy that made him feel alienated.
One possible reason could be a difference in ideology. Maybe Peter Parker feels that Stark's methods are too extreme or not in line with his own moral compass. For example, Stark might be involved in some projects that Peter doesn't agree with, such as using certain unethical technologies in the pursuit of justice.
One reason could be that he feels ostracized. Despite his efforts to protect Konoha, some villagers still look at him with suspicion because of the Nine - Tails inside him. He might just get tired of constantly having to prove himself and decide to leave in search of a place where he can be accepted for who he is.
One possible reason could be a difference in ideology. Maybe Spider - Man has a more local - friendly approach to crime - fighting while the Avengers are often involved in large - scale, global threats. He might feel that his skills are better utilized in his own neighborhood, like Queens, rather than in the grand, often cosmos - level battles of the Avengers.
One possible reason could be ideological differences. Maybe she discovers some practices within the Order that go against her moral compass. For example, if the Order starts to use unethical magic in their fight against the dark forces, Hermione, with her strong sense of right and wrong, might not be able to tolerate it and decide to leave.
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.
Perhaps he feels pressured in Konoha. Having a kid adds more responsibility, and he might think that leaving Konoha will give him the space he needs to grow as a father and a ninja without the constant eyes of the village on him. He could be seeking a place where he can train his kid in a different way, away from the traditional Konoha methods.
One possible reason could be the way the Dursleys treated him. They were cruel, made him live in a cupboard, and constantly belittled him. This kind of mistreatment over the years could easily lead to hatred. Another aspect could be the lack of connection to his wizarding family that he was not aware of for a long time. He might feel angry at his parents for leaving him in such a situation without any real protection from the Dursleys.
One possible reason could be to escape the danger in England. Given the dark forces that were at play, especially Voldemort's rise, they might have felt it was safer to leave. Another reason could be for a fresh start. Maybe they wanted to explore new magical cultures and communities in other parts of the world.
Perhaps she has a new mission or calling that takes her away from Camelot. It could be something related to the greater magical world. For example, there could be an ancient magic that is in danger of being lost or misused, and she is the only one who can save it. Since Camelot has been rather unkind to magic in general, she might feel that she has to leave to fulfill this important task.