Perhaps after facing too much betrayal from the village he loves. If the villagers turned on him despite all his efforts to protect them, it might lead him to close his heart in self - defense, not wanting to be hurt again. He's always tried so hard to gain their acceptance and if that was all for naught, it would be a huge blow.
It could also be due to the constant pressure and expectations on him. He's always had to be the hero, the one to save the village. If he feels he's constantly failing or not living up to those expectations, he may close his heart to protect himself from that self - doubt and disappointment. In the fanfic, perhaps there are new threats to the village that he feels he can't handle, and this leads to him shutting down emotionally.
Well, it might start with Naruto distancing himself from his friends. He stops going on missions with them, and he doesn't show up at their usual gathering places. For instance, he might avoid Ichiraku Ramen, which was always a place of connection for him and his friends.
Self - reflection is key. If Naruto takes the time to really look at himself and understand why he closed his heart in the first place, he can start to work through those issues. Maybe he realizes that closing his heart is only causing him more pain in the long run and that by being open, he can experience the full range of human emotions again, both the good and the bad.
I recommend " Happiness on the Left " and " Contracted Boyfriend is the President " to you. " Happiness on the Left " was about a beautiful and cold girl who sealed her emotions in order to forget the pain of the past, until she met a man who closed his heart. " Contracted Boyfriend is the President " was about the male protagonist, Gu Han, who had learned to shut himself off due to the mysterious death of his parents. It was not until the appearance of the female protagonist, Xia Lewei, that his heart changed. Both novels described the protagonist's experience of emotional trauma and sealing off his emotions. I hope you like these recommendations!
One possible reason could be to protect his friends. He might think that by sacrificing his heart, some great danger or enemy would be vanquished, thus ensuring the safety of those he cares about. For example, if there was a powerful curse that could only be broken by his heart's sacrifice.
Another reason might be related to some sort of experiment gone wrong. Peter is often involved in scientific research. If there was an accident in his lab, perhaps with a new type of spider - related serum or some other experimental technology, it could have unforeseen consequences on his heart and lead to it stopping.
Perhaps the death of a key person in his life could lead to him abandoning his dream. If someone like Iruka or Kakashi, who were important mentors to him, died in a way that made Naruto question the purpose of his dream. He might think that without their support and guidance, his dream was no longer worth pursuing.
One possible reason could be the burden that comes with his heritage. Naruto's heritage is filled with great power and responsibility. Maybe he feels that he can't live up to the expectations of his clan or predecessors, so he rejects it.
One possible reason could be to protect her. Naruto might believe that by leaving, he can draw danger away from his sister. For example, if there are powerful enemies targeting him, he doesn't want his sister to get caught in the crossfire.
Maybe he's been influenced by someone who has a different view of heritage. For example, if he meets a character who believes that one should create their own identity rather than relying on heritage, Naruto could be swayed. Another possibility is that he has some sort of resentment towards his heritage. Perhaps there are things in his family's past or in the traditions associated with his heritage that he doesn't agree with or is ashamed of.