One reason could be to explore a different side of Naruto's character. In the original series, he's so warm - hearted, but by making him cold and distant, the writer can show how different events could have shaped him in an alternate way. Maybe he faced more hardships or betrayals that led to this change.
Perhaps the writer wants to show a more 'realistic' or jaded version of Naruto. After all he has been through in the original story, some might think that it's unrealistic for him to always be so positive. So, making him cold and distant could be a way of showing how the ninja world could have worn him down over time.
Well, it's hard to say it's a widely popular genre on its own. The Naruto fandom has a vast range of fanfic types. While the idea of Naruto being a cold and distant shinobi might attract some readers who like exploring different character portrayals, it's not as mainstream as some other common themes like Naruto achieving new powerful jutsus or his relationships with other main characters. There are so many different takes on Naruto in fanfics, and this rather 'edgy' or different view might only appeal to a niche group within the large Naruto fan base.
Perhaps he has trust issues. He might have been betrayed in the past, so now he keeps everyone at a distance to avoid getting hurt again.
Maybe he realizes that the shinobi world is too corrupt. There could be political intrigue and power struggles that go against his ideals. He has always fought for peace and unity, but if he sees that the system is beyond repair, he might choose to quit. Also, he could have a family situation that requires him to step away from the shinobi life. For instance, if he has a child with special needs that demands his full attention.
Another reason could be the influence of a powerful and dark entity. If Naruto is somehow possessed or influenced by a dark chakra or a malevolent spirit, it might change his personality completely. He could start to push away people who care about him because the entity wants him to isolate himself and be more vulnerable to its control.
One possible reason could be a major betrayal. Maybe someone he trusted deeply, like a close friend or a mentor, turned against him. This would make him lose faith in people and become cold as a form of self - protection.
Some writers could be exploring the 'what if' scenarios. They might wonder what would happen if Naruto had a different upbringing or faced different traumas that could turn him into a cold - hearted killer. It's a way of delving deeper into the character's potential under extreme circumstances. Also, it could be a form of creative rebellion against the typical positive portrayal of Naruto in the main series.
It could be due to a new jutsu he's trying to master that has some side effects on his emotions. Maybe it's a powerful ice - based jutsu that's making him cold in nature as well.
One reason could be that she might have faced too much trauma from battles. Constant fighting, seeing friends get hurt or die, could have taken a toll on her mental health, leading her to quit.
One possible reason could be a major betrayal he experienced. Maybe someone he trusted deeply, like a close friend or a mentor, turned against him. This would make him more guarded and distant to avoid getting hurt again.
One reason could be that Naruto has been through a lot and is emotionally drained. He might see Tsunade as a source of comfort and stability, so he calls her mom. His coldness could be a defense mechanism to hide his vulnerability. Another reason could be that he has seen how Tsunade cares for others in the village, and he subconsciously wants that kind of motherly love for himself.