In some fanfics, he might leave to protect the magical world. He realizes that his presence is attracting too much trouble and evil, so by leaving, he hopes to keep the world he once loved safe.
One reason could be that he's tired of the constant danger and battles. He might just want a normal, quiet life away from all the magic chaos.
Perhaps he feels betrayed by some in the magical world. Maybe some secrets were revealed that made him lose faith in the magical community, so he decides to leave.
In some cases, he could be leaving in search of a greater purpose. He might feel that there are things he can learn or achieve outside the wizarding world that will ultimately help the wizarding community more. He may also leave because of some prophetic vision or a feeling that he doesn't belong there anymore, that his path lies elsewhere, away from the familiar magical realm he grew up in.
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 that Harry discovers some secrets about Dumbledore that make him lose trust. For example, if he finds out about Dumbledore's past mistakes or hidden agendas that he doesn't agree with. Another reason might be that Harry feels he can better protect his friends and fight Voldemort on his own terms, away from Dumbledore's influence and strategies which he may start to question.
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 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 a falling - out with his friends in Gryffindor. Maybe there was a misunderstanding that grew so large that he felt he couldn't stay. Another possibility is that he might be lured away by some mysterious force or character who convinces him that Gryffindor is no longer the right place for him. For example, if a new group claims to have a better way to fight Voldemort and they are from a different 'house' in this fanfiction concept.
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.
One possible reason could be a conflict within the team. Maybe Naruto felt misunderstood by Sasuke or Sakura. Another reason might be a mission gone wrong where Naruto blames himself and decides he's a liability to the team. Or perhaps he gets an offer from another group that he believes can help him achieve his goals faster.
Another reason could be that they had some sort of premonition or prophecy that indicated the Dursleys' place was a crucial part of Harry's future protection. Maybe a seer told them that for Harry to grow into the wizard he needs to be, he has to endure the Dursley experience first.
One reason could be that he's tired of the constant danger and battles. After all he's been through so much, from fighting Voldemort to dealing with various dark wizards. Another reason might be to protect the magical world itself. He might think that his presence attracts too much trouble and by leaving, he can keep the world safer. Also, he could be in search of a normal life. Growing up without his parents and always in the spotlight in the magical world, he may long for a simple, non - magical existence.