There are several reasons. He might be afraid of getting too close to people because he's lost someone important and is scared of more loss. It could also be that he's discovered some new power within himself that he doesn't understand and is worried about hurting others if he gets too involved. Additionally, there could be some political intrigue in the village. Maybe there are some people who are plotting against him and he's keeping to himself to avoid being a target or putting others in danger.
In the fanfiction, Naruto could keep to himself due to a number of factors. For starters, he may have had a falling - out with a close friend. This emotional upheaval could make him want to isolate himself. He might also be dealing with self - discovery issues. As he grows older, he could be realizing things about his past, like his parents' real story, that are so overwhelming that he needs time alone to process. Moreover, he could be facing external pressures from the ninja world, such as being targeted by a new enemy group, and he's keeping to himself to protect those around him.
One possible reason could be extreme guilt. Maybe he fails to protect his friends in a major battle and feels so responsible that he can't bear it. For example, if Sasuke were to die because Naruto couldn't reach him in time, Naruto might sink into such deep guilt that he sees no way out but death.
One possible reason could be the burden of his past. Naruto has faced a lot of hardships and the memories might be overwhelming him, making him want to be alone. For example, the loss of his master Jiraiya might have left a deep scar, and he might feel that isolating himself is the only way to deal with that pain.
Maybe he feels a sense of guilt. For example, if something bad happened in the village that he blames himself for, even if it wasn't really his fault. He could exile himself as a form of self - punishment. He has always been very hard on himself when it comes to the well - being of the village and his friends.
Yes, such fanfictions exist. In these fictional scenarios, Naruto might kill himself because of Sasuke for several reasons. Naruto's whole life has been centered around Sasuke to some extent. If he feels that he has failed in his mission to bring Sasuke back to the right path and that the situation has become so dire that the only way to atone for his 'failure' is through his death, he might do it. Also, if Sasuke were to cause harm to someone Naruto holds dear, like the village or his other friends, and Naruto feels powerless to stop him in any other way, he could choose to end his own life as a last resort.
One possible reason could be the loss of his friends. Naruto values his friends deeply, and if he were to lose someone like Sasuke again, it would be devastating. Another reason might be the pressure of being the Hokage. The responsibilities are huge, and he might feel like he's not living up to the expectations of the village. Also, memories of his lonely childhood could resurface, making him feel sad and alone, leading to him crying himself to sleep.
Another reason might be related to his pursuit of power. If in the fanfiction, Naruto is constantly facing stronger enemies and he believes that his current strength is insufficient to protect those he loves, he might attempt to sacrifice himself in a desperate attempt to gain more power, like through some dangerous and forbidden jutsu.
One possible reason could be for a mission. Maybe the mission requires infiltrating a female - only place or getting close to a female - only group, so Naruto decides to turn himself female to better complete the task.
It could also be due to a sense of isolation. If all his friends turned against him for some reason, perhaps being wrongly accused of a heinous crime. Naruto, who values his bonds deeply, might find such a situation unbearable and choose to end his own life.
Maybe he was trying to escape some disease or a dangerous situation. Or, it could be part of a scientific experiment.
One possible reason could be to hide his true identity. In the world of fanfiction, there might be threats or situations where he doesn't want to be easily recognized as Harry Potter. Maybe he wants to start anew, away from the fame and the constant danger associated with his name.