Perhaps Robin leaves in fanfiction to pursue his own solo journey of self - discovery. He might feel that he has grown as much as he can within the Titans and needs to explore on his own to fully understand his capabilities and purpose. He could be seeking new challenges away from the group dynamic.
One possible reason could be a conflict within the team. Maybe there are differences in opinions on how to handle villains or protect the city. For example, if Robin has a more aggressive approach while the other Titans prefer a more diplomatic way, it could lead to a rift and his departure.
One possible reason could be that he feels he has fulfilled his purpose there. Maybe he has achieved certain goals related to protecting Harry Potter secretly, and he decides it's time to move on to other things. For example, in some fanfictions, he might leave to explore his own family history or to further his research on the Dark Arts in a more secluded place.
One possible reason could be to seek new challenges. After achieving so much in the Elemental Nations, he might feel the need to explore areas where his skills can be tested in different ways.
Maybe he leaves because of a conflict with the Hogwarts staff. If the new headmaster or some teachers start implementing rules that Harry believes are wrong or unfair, like restrictions on defensive magic classes, he might decide to leave in protest.
One reason could be a mission. Maybe he gets a top - secret mission that requires him to leave the village under Tsunade's leadership without her direct involvement. So he has to leave without much notice.
One reason could be to find peace. After all the chaos and war in England, they might just want to get away from it all and start fresh somewhere else.
One possible reason could be false accusations against him. Maybe he's wrongly blamed for a crime he didn't commit, like being framed for a major attack in the village. This would lead the village leaders to force him out to maintain 'peace' and security.
Some people might create it out of a desire to explore darker or more complex storylines. They may think it adds an edgy or different perspective to the relationship between the two characters.
Love at first sight could be a reason. Maybe they meet for the first time during a team - up and there's an instant spark between them that leads to a kiss.
Some writers might be interested in exploring the darker side of human nature. They could use the well - known Batman - Robin relationship and twist it to show how power dynamics can go wrong. It's also possible that they want to create shock value to get more attention for their work.
Another reason might be that Naruto is seeking a greater purpose. In the fanfiction, his 'godlike' state could make him realize that there are threats out there that he can't face while being tied to Konoha. He may leave to protect the world on a larger scale, believing that he can be more effective away from the village.