Perhaps he has had an experience that made him more introspective. Like a difficult mission where he saw the darker side of things. This could have led him to be quieter as he contemplates the meaning of his life and his relationships with his friends. He may be afraid that if he speaks too much, he'll say something wrong or reveal too much of his new, more complex thoughts.
One reason could be that he's going through some internal struggles. Maybe he has secrets he doesn't want to share. For example, he might be worried about burdening his friends with his own problems.
It might make his friends a bit worried at first. They're used to the more outspoken Naruto. But it could also draw them closer as they try to figure out what's wrong.
One possible reason could be that Naruto is afraid of endangering his friends. He has many powerful enemies, and he might think that by staying away from his friends, they won't be targeted. For example, Orochimaru was always after Sasuke, and Naruto might fear that his closeness to Sasuke could put Sasuke in more danger.
One possible reason could be envy. If Naruto suddenly gains a great deal of power or attention in the fanfiction, his friends might feel left out or jealous. Another reason could be a misunderstanding. Maybe Naruto is framed for something he didn't do, and his friends believe the false accusations without giving him a chance to explain. Also, in some fanfiction, a change in Naruto's personality, like becoming overly aggressive or secretive, could lead to his friends growing to dislike him.
One reason could be to seek greater power. In fanfiction, the world outside Konoha might be seen as having more opportunities for growth. They might want to train with different masters or learn unique jutsu not available in Konoha.
Since Naruto is quiet, he may not be as quick to make friends as in the original story. But the friends he does make could be more special. They would likely have to make an effort to get to know him better. His quietness could also make his friends more protective of him in a sense. They might see him as more vulnerable, and so they'll be there to stand up for him when needed. And as he gradually opens up to them, it could create a very deep and meaningful friendship that's based on a true understanding of each other.
One possible reason could be jealousy. Maybe Naruto has been getting more attention from the village elders or teachers because of his potential. His friends, feeling left out or overshadowed, might start to ignore him out of envy.
In some fanfictions, Naruto might be hated by his friends due to a misunderstanding. For example, if there's a false rumor spread about him being a traitor. Another reason could be his sudden increase in power that makes his friends jealous. Maybe he accidentally harms one of them during a training session and they hold a grudge.
Maybe his friends betrayed his secrets. In many fanfictions, a friend might accidentally or deliberately let out something Harry had confided in them, like his fears about Voldemort or his feelings for a certain girl. This could shatter his trust.
One possible reason could be jealousy. Maybe Harry has achieved something great and his friends can't handle his success. For example, if he gets a very important role in a magical competition, his friends might feel overshadowed. Another reason could be a misunderstanding. Harry might be wrongly accused of something, like being in league with the dark side, and his friends, in their fear or haste, abandon him without giving him a chance to explain.
One reason could be a misunderstanding. Maybe there was a miscommunication between Harry and his friends, and he felt so hurt or angry that he decided to leave. For example, if his friends wrongly accused him of something he didn't do.