One possible reason could be that he realizes his feelings are unrequited. In the fanfiction, perhaps Lily has made it clear she doesn't have the same feelings for James, so he gives up. Another reason might be external factors. For example, there could be some sort of danger or situation that makes James think he needs to step back from Lily for her own good.
Maybe James Potter gives up on Lily in the fanfiction because he has grown as a character. He might have come to understand that his pursuit of Lily was more of an obsession than true love. So, he decides to let her go and focus on his own personal growth and development. It could also be that he has new responsibilities or a new mission in the story that requires him to give up on his feelings for Lily.
It could also be due to some external factors. Maybe there are threats from dark forces that he doesn't want to involve Lily in. If he is getting deeper into the dark arts or some dangerous situations, he might choose to give up on her to protect her from the potential harm that could come her way because of his associations.
One possible reason could be that J.K. Rowling, the author of Harry Potter, has strict control over the franchise and might have made some regulations that made it difficult for him to continue. Another reason could be that he simply lost interest in writing fanfiction about it. Maybe he wanted to focus on creating his own original stories instead.
Well, the story could take a turn towards James finding a new purpose. Maybe he devotes himself to his studies or some magical research. He might also start to form new friendships or alliances that were previously overlooked because of his focus on Lily.
In the fanfiction, perhaps differences in their future plans might lead to divorce. Maybe James wants to move to another city for a job opportunity, but Lily doesn't want to leave her family and friends behind. So, this fundamental difference can drive them apart.
One possible reason could be an alternate universe scenario. Maybe in that fanfic's universe, there are dark forces at play that make Lily and James act out of character and ignore Harry. It could also be a plot device to create a more angsty or dramatic story, where Harry has to face challenges without the support of his parents.
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.
It could also be that he is tired of the false expectations created by fanfiction. In fanfiction, his character might be portrayed in ways that don't match who he really is. He wants to be true to himself and his ninja way, so he gives up fanfiction.
One possible reason could be a lack of time. Maybe Ash has become too busy with other aspects of life, like work or studies, and can't find the time to write fanfiction anymore.
Perhaps he simply lost his creative inspiration. Fanfiction writing often requires a constant flow of new ideas, and if he's been through a lot of stress or has had some personal issues, like his complex relationship with Lily still haunting him, it could dry up his well of creativity.
It could be an exploration of the darker side of human nature. Writers may use these characters to show how relationships can go horribly wrong, even if it's not in line with the original story.