One reason could be that he experiences a great loss, like the death of a dear friend. In many fanfictions, when Naruto loses someone important, it shatters his positive outlook, and he stops smiling as a result.
Maybe he discovers a dark secret about the village he loves so much. For instance, if he finds out that the village elders have been involved in unethical practices, it could make him lose his faith and his usual cheerfulness.
It could also be due to a broken heart. If he has feelings for someone and they reject him or are in danger because of him, Naruto might become so burdened that he stops smiling. In a fanfiction, his love life can be a complex and emotional aspect that can greatly affect his mood.
One possible reason could be a misunderstanding. Maybe Sakura said something that Naruto misinterpreted as being hurtful or dismissive of his feelings. For example, if she made a comment about his abilities in a negative way during a crucial moment, it could lead to him distancing himself.
In the fanfiction, Naruto could stop chasing Sasuke because he has a moment of self - realization. He might understand that his own identity and purpose in life are not solely defined by bringing Sasuke back. He could find new goals and dreams that don't involve Sasuke. Also, there could be a major event in the ninja world that makes Naruto shift his focus away from Sasuke, like a new and powerful enemy that threatens the entire village or region.
Maybe he achieved something he had long desired. For example, he finally grew the biggest tomato in his garden after years of trying.
Perhaps they are jealous of others in the community. So, they put on insidious smiles while secretly planning to undermine those they envy. They might smile while spreading false information about their rivals. Another reason could be that they have some sort of power - hungry ambition. They use their smiles to get close to people in positions of power or influence in order to manipulate them for their own gain. They act friendly with those smiles but have ulterior motives.
One possible reason could be that Naruto finally realizes Sakura's true feelings for Sasuke are unwavering. He might come to understand that his constant pursuit is fruitless and decides to respect her feelings and stop asking her out.
One possible reason could be that Naruto has finally realized that Sakura's heart has always been with Sasuke. After years of chasing after her and getting constantly rejected or overlooked, he just gets tired and decides to move on. He might start to focus more on his own personal growth and the other relationships in his life that are more reciprocal.
One reason could be that he gets tired of the constant traveling and battles. It's a very physically and mentally exhausting lifestyle. Another might be that he experiences a major loss, like losing his beloved Pokémon, which makes him lose his passion for training. Or perhaps he discovers a new interest that he finds more fulfilling than Pokémon training.
It could be due to the failure of protecting his village or his loved ones. Naruto has always been about protecting those important to him. If he fails at that, his anger is understandable.
One reason could be to show a different side of Naruto. The original Naruto is very social and friendly, but in a fanfic, making him antisocial can create a contrast. Maybe the writer wants to explore how his friends would react if he suddenly changed.
One reason could be the fear of losing his friends. Naruto has always been very protective of his comrades. In some fanfictions, he might face a situation where he is on the verge of losing them, like a powerful enemy threatening to kill them all. This would make him afraid as his friends are his most precious things.