It could be that they are part of a cult or some sort of secret society. Their insidious smiles are a form of communication within the group. They might be smiling to signal to each other during their interactions with outsiders. Also, they could be using their smiles to lure in new members. They present a false sense of friendliness with their smiles to make people lower their guards and become interested in what they have to offer, which is actually something harmful or unethical.
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.
Once upon a time, there was a family that always had these insidious smiles. They lived in a big old house on the outskirts of town. The parents would smile in a strange way whenever they interacted with the neighbors. It turned out they were hiding a big secret. They had stolen a large sum of money years ago and were living in fear of being discovered. Their smiles were a way to cover up their guilt and nervousness.
Maybe he achieved something he had long desired. For example, he finally grew the biggest tomato in his garden after years of trying.
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.
One possible reason could be a divergence in values. Maybe in the fanfiction, Naruto discovers that his family was involved in some dark or unethical deeds that go against his principles. For example, if his family was secretly working with an enemy organization that Naruto has been fighting against. Another reason might be a sense of betrayal. If his family had made decisions in the past that put Naruto in a difficult or dangerous position without his knowledge, he could feel the need to denounce them. Also, it could be due to a misunderstanding. False information might be spread about his family, leading Naruto to believe they are not what he thought they were.
In some fanfictions, Harry might leave his family because of a prophecy or a special mission. He might feel that he has to go it alone in order to fulfill this prophecy or complete the mission successfully. His family might not understand or be able to support him in this, so he decides to leave. Also, there could be jealousy or resentment within the family that makes the environment unbearable for Harry.
In some fanfictions, Naruto might abandon his family due to a false prophecy that convinces him he's a danger to them. He may think that by leaving, he can protect them from harm that might come his way because of his past or enemies.
One possible reason could be jealousy. Maybe Naruto has some special abilities or a prophecy surrounding him that makes his family members, who might be less favored, jealous. For example, if there's a prophecy that Naruto will be the most powerful ninja in the village, his siblings or parents might envy that future for him and start to hate him out of their own insecurities.
It could also be due to a sense of betrayal. If he found out some dark secrets about his family, like his father being involved in something bad that he couldn't forgive. This could make him turn his back on his family and not care about them at all.
In some fanfics, it could be related to a prophecy. His family might believe that Naruto is going to bring about a great disaster according to a misinterpreted prophecy. So, they turn on him in an attempt to prevent the prophecy from coming true, even though they are wrong in their understanding.
One possible reason could be some kind of dark magic or jutsu that has affected his family's memories. Maybe an enemy cast a spell specifically to make his family forget about him to weaken him.