It could be Sasuke. If Sasuke has continuously pushed Naruto away or done something extremely hurtful, it might have made Naruto feel tired of the whole situation. Since Sasuke is such an important person in Naruto's life, his actions can have a huge impact on Naruto's mental state.
The village elders might be the reason. They could have put too much pressure on Naruto, always expecting him to solve every problem. Their unending demands and lack of real understanding of Naruto's feelings could lead to him being tired of living under their expectations.
Well, one possibility could be that Naruto has faced so many hardships and losses in the fic that he's simply mentally and physically exhausted. Maybe he's lost too many friends in battles, or the constant pressure of being a hero has finally gotten to him.
Maybe Orochimaru could be the main antagonist in this fanfiction. He might see Naruto's state of being tired of pretending as an opportunity to manipulate him. Orochimaru is always looking for ways to gain power and a vulnerable Naruto could be a target for him. He could try to convince Naruto to turn to the dark side by preying on his insecurities that he is now showing instead of hiding them as before.
In this fanfiction, Naruto might leave because of the political situation in Konoha. There could be factions that he doesn't agree with or that are causing unrest. Naruto, being someone who values peace and unity, might find it difficult to deal with the internal politics and decides to leave in order to find a more harmonious place or to figure out a solution away from the chaos.
Another reason could be the lack of trust he feels from his teammates in some fanfiction scenarios. If Sasuke is constantly keeping secrets or Sakura is doubting Naruto's abilities, it can make him feel isolated within the team and eventually tired of being part of it.
In this fanfiction, perhaps Naruto is tired of pretending that he doesn't feel the weight of being the hero. He could start to show his true emotions, like his insecurities and doubts. This could lead to some interesting interactions with his friends. For example, Sasuke might be one of the first to notice the change in Naruto and they could have a heart - to - heart conversation about their past and future. And it might also cause a stir in the village as people are used to seeing Naruto in a certain way.
It could be Naruto himself. Imagine Naruto getting to relive a part of his life, like his early days in the academy. He could use his future knowledge to avoid some of the hardships he faced or be even better prepared to face the challenges that came his way.
It might also be a new character created specifically for the fanfiction. This character could be someone who has infiltrated the Hidden Leaf Village, living behind a mask of false loyalty while having their own secret motives and goals.
Sakura would be a likely candidate. She is a medical ninja, and she has always cared about Naruto. She would use her medical skills to diagnose his condition and try to heal him. She has healed many of her comrades before, so she would be very capable of helping Naruto.
Well, he might encounter unique creatures. Maybe he'll befriend a magical deer that helps him find rare herbs for his ninja training. He could also build a really cool tree - house as his hideout, and use natural elements like vines and branches to set up traps for any potential enemies.
One possible reason could be that Naruto is feeling lonely. Despite having friends, there are times when his past as an orphan and the years of being shunned by the villagers resurfaces, making him feel isolated. Kakashi finding him in such a state might then lead to a heart - to - heart conversation about Naruto's inner struggles.