Perhaps in the fanfiction, Josh has a different set of values or goals that conflict with Castle's. If Josh is pushing for changes or actions that go against what Castle believes in, it might drive Castle away. For instance, if Josh is more focused on power and wealth and is willing to do unethical things to achieve it, and Castle is an ethical person, he would leave to distance himself from such behavior.
It can create a sense of loss and change the dynamic among the characters. The other characters might have to adjust to Castle's absence, and it could lead to new alliances or rivalries.
Maybe in the fanfic, Beckett got a new job opportunity in a far - away place. This could make Castle think she's leaving him.
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.
One reason could be that he feels a sense of being out of place. Maybe new campers have come in and he no longer feels as important or needed. He might also leave in search of a greater purpose. In some fanfictions, he could be on a quest to find something that he believes is crucial for the safety of the camp, but he has to leave the camp itself to do it.
One reason could be that she feels she has outgrown the team. Maybe she has achieved all she can within the Avengers and wants to explore new challenges on her own. For example, she might want to focus on solo missions that are more in line with her personal moral code.
One reason could be that Naruto might feel he has outgrown Jiraiya's teachings in the fanfiction. He may want to explore his own path independently. Another possibility is that Naruto gets involved in a mission or a situation where he has to part ways with Jiraiya for the greater good. For example, if there is a threat in a far - off land that only Naruto can handle, and Jiraiya has other commitments elsewhere.
Perhaps it's due to personal relationships. He might have had a falling - out with one or more of the other Avengers. Like if there were misunderstandings or betrayals. In some fanfictions, a conflict with Steve over leadership or ideological differences could lead to Tony leaving the team.
One reason could be to seek more power. In some fanfictions, Naruto might feel that the Leaf doesn't offer enough resources or training for him to reach his full potential. So he decides to leave to find a place where he can grow stronger.
One reason could be ideological differences. Maybe she discovers that the Order's methods don't align with her own sense of justice. For example, if the Order starts to use unethical magic in their fight against the Dark forces, Hermione, with her strong moral compass, would not stand for it.
One possible reason could be a difference in their goals. Naruto is from the Hidden Leaf Village with a mission to protect it and promote peace there. Rias, on the other hand, has her own responsibilities in the Underworld. Their conflicting duties might lead to Naruto leaving.