One reason could be betrayal by his friends. For example, if someone he considered a close ally made a deal with his enemies behind his back, it would shatter his trust. Another might be lies from the village leaders he respected. If they hid important information or manipulated him for their own gains, Naruto would start to question who he could trust.
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.
It could be due to miscommunication. In the fanfiction, perhaps there were misunderstandings within the family. For example, Naruto might have overheard something that made him think his family didn't have his best interests at heart, but in reality, it was all a big mix - up. So, he gradually lost trust in them.
One possible reason could be the loss of his friends. Naruto deeply values his relationships, and if something were to happen to his friends like Sasuke leaving again or getting seriously injured, it would shatter his happiness. Another reason might be if he fails to protect the village. Naruto has always seen himself as the protector of Konoha, so if he fails in this duty, his sense of purpose and happiness would be lost.
Another reason might be related to a plot device in the fanfiction. The author could be using the loss of the mask to create a turning point in the story. It might lead to new relationships being formed or old ones being tested. For instance, his friends might react in different ways when they see his face without the mask, and this could drive the story in different directions.
Naruto could become bitter. Losing trust, especially in those he once held dear, can make him view the world in a more negative light. He may no longer believe in the good of people so easily and might be more cynical. For example, he might not be as quick to offer help or friendship as he used to be.
One possible reason could be past betrayals. Maybe he has been let down by people he thought were friends in the fanfiction world. For example, if someone he trusted shared his secrets which led to him getting into trouble.
One possible reason could be that Naruto has had some past experiences within the team that made him feel betrayed or let down. Maybe there was a mission where he thought his teammates didn't have his back. Another reason might be that there are secrets among the team members that Naruto has discovered, and these secrets make him question their loyalty. For example, if one of his teammates was hiding a connection to an enemy, Naruto would naturally start to lose trust.
It could also be due to Sasuke's secrecy. If Sasuke is constantly hiding things from Naruto, like his real intentions or new powers he's acquired, Naruto would naturally start to feel that he can't trust him. Trust is built on honesty and openness, and if Sasuke is being cagey, Naruto's trust in him will fade.
His lack of trust could lead to a lot of misunderstandings. For instance, if someone tries to help him but he misinterprets it as a malicious act because of his distrust. This can create a cycle of more distrust as others start to feel frustrated with him.
One possible reason could be past decisions made by Sarutobi that Naruto didn't agree with. For example, Sarutobi's handling of the Uchiha situation. Naruto, who values friendship and fairness deeply, might have seen this as a sign of Sarutobi's inability to do what's right, thus leading to his distrust.