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.
It could be due to a prophecy or a pre - determined path in the fanfiction world. If there is a prophecy that says he must distance himself from Gryffindor to fulfill a greater purpose, like uniting all the houses against a common enemy in a way that requires him to be an outsider. Or maybe he has a vision that leads him to believe leaving Gryffindor is the only way to protect his friends and Hogwarts as a whole.
Perhaps he feels that Gryffindor has become too restrictive. In some fanfictions, there could be new rules or a change in the house's atmosphere that doesn't align with Harry's values or goals anymore. He might also leave because he discovers a long - lost family connection to another group or place that makes him question his loyalty to Gryffindor.
One possible reason could be a major disagreement with the Gryffindor house leadership or some of the students. Maybe he feels that they are not living up to the values he thought they held. Another reason might be that he discovers some dark secret within Gryffindor that makes him uncomfortable and he decides to leave.
One reason could be that he doesn't like the spotlight that comes with being a Gryffindor. In the original story, Gryffindors are often in the limelight, and in fanfiction, he might be tired of all the attention.
One reason could be that the fanfic distorts the characters he knows well from Gryffindor. For instance, if it changes the personalities of his friends in Gryffindor in a way that doesn't seem right to him.
Another reason could be a misunderstanding. Maybe he overhears something Harry and Hermione say and misinterprets it as them not wanting him around anymore. So, in a fit of anger and hurt, he leaves. This can lead to a lot of angst and drama in the fanfiction as the other characters then have to figure out how to get him back.
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.
In some fanfiction, Harry leaves to seek more powerful magic. He realizes that the magic in Britain is somewhat limited in the fight against Voldemort. So he travels to other places, like ancient magic - rich regions in Asia or Africa, to find a way to defeat Voldemort. He hopes that by leaving, he can gain the upper hand in the battle.
One reason could be a personal betrayal. Maybe a close friend from Gryffindor betrays Harry, and this makes him start to dislike the whole house. It's like when one bad apple spoils the bunch in his eyes.
In the fanfiction world, it could be a prophecy. There might be a prophecy that says Harry's presence in Ginny's life will bring great harm to her or the wizarding world. So, for the greater good, Harry leaves. He believes that by sacrificing his relationship with Ginny, he can save her and everyone else. It's a very noble but also a very heart - breaking decision.
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 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.