Another reason might be that Naruto has been influenced by external forces. Maybe he has made some bad friends or been involved in some situations that made him think being cold to his family was the right thing. He could have been brainwashed into believing that his family was holding him back from achieving something greater.
One possible reason could be a feeling of betrayal. Maybe he found out some long - hidden family secrets that made him lose trust in his family. For example, if he discovered that his parents had made a deal with some dark force in the past that had negative consequences for him or the village.
One possible reason could be that he has faced a lot of misunderstandings within the family. Maybe his family has certain expectations that he can't meet, and over time, this has led to his coldness. For example, if his family doesn't understand his dreams or ambitions, Naruto might feel alienated.
One possible reason could be a misunderstanding. Maybe there were some miscommunications in the family that made Harry feel alienated, so he became cold. For example, if his family misunderstood his intentions in certain matters, like his career choice or relationships, it could lead to this situation.
Well, his friends would be really confused at first. They're used to Naruto's warm and energetic self. Sasuke might be a bit more standoffish in return, thinking Naruto has changed for the worse. Sakura would probably try to get to the bottom of it, constantly pestering Naruto to find out what's wrong. His other friends like Shikamaru might just give him space at first, waiting for him to come around on his own.
It could also be due to some external influence. Perhaps an enemy manipulated Naruto's mind in the fanfiction, making him see his family in a negative light. This enemy might have used some form of mind - control or false memories to turn Naruto against his family.
One possible reason could be that Naruto has his own unrequited love for someone else and he is trying to distance himself from Sakura to avoid leading her on. For example, if he has deep feelings for Hinata, he might be cold towards Sakura to make his own feelings clear.
One possible reason could be a divergence in values. Maybe in this fanfiction, Naruto's family holds values that are completely opposite to what Naruto believes in. For example, if his family is shown to be more focused on power and status within the Justice League world, while Naruto values friendship and helping the weak.
One possible reason could be that in the fanfiction, his family might have been portrayed as overly strict or controlling. Maybe they forced him into a certain path he didn't want to take, like a particular ninja training regime that Naruto found too rigid.
One reason could be a great loss. Maybe he lost someone really important like Jiraiya in a more tragic way that made him shut down. Another could be that he discovered some dark secret about the village that made him lose faith in everyone.