One reason could be that he's going through some internal struggles. Maybe he has secrets he doesn't want to share. For example, he might be worried about burdening his friends with his own problems.
Since Naruto is quiet, he may not be as quick to make friends as in the original story. But the friends he does make could be more special. They would likely have to make an effort to get to know him better. His quietness could also make his friends more protective of him in a sense. They might see him as more vulnerable, and so they'll be there to stand up for him when needed. And as he gradually opens up to them, it could create a very deep and meaningful friendship that's based on a true understanding of each other.
It might make his friends a bit worried at first. They're used to the more outspoken Naruto. But it could also draw them closer as they try to figure out what's wrong.
One possible reason could be that Naruto is afraid of endangering his friends. He has many powerful enemies, and he might think that by staying away from his friends, they won't be targeted. For example, Orochimaru was always after Sasuke, and Naruto might fear that his closeness to Sasuke could put Sasuke in more danger.
A good quiet Naruto fanfiction is 'Naruto's Silent Solitude'. In it, Naruto often retreats to a quiet place in the forest outside Konoha. Here, he reflects on his life, his loneliness, and his dreams. It's a more introspective take on his character. Then there's 'Quiet Whispers in the Leaf'. This fanfic delves into the unspoken emotions between Naruto and his friends, like how they communicate without words in certain situations. It's a heartwarming read.
The 'naked around his friends story' could be a variety of things. It might be a comical story where a guy accidentally showed up naked in front of his friends. For example, he thought he was alone in a place where his friends were also present, like a holiday house. He might have come out of the shower thinking everyone was at the beach, and there he was, naked among his shocked friends. Or it could be a more serious story about body acceptance, where a person decided to be naked around his friends as a way to overcome insecurities and promote a sense of openness and trust within the group.
In a fanfiction scenario, Naruto might be in a calm and quiet forest glade. He is there to meditate and also practice his slashing attacks. As he focuses in the quietness, he realizes that his slashing moves can be combined with his natural energy absorption. So he starts to develop a new type of slashing move that is powered by both his chakra and the natural energy around him. This makes his slashes more powerful and also gives them a unique greenish glow. His friends are amazed when he shows them his new technique later.
Typically, Naruto is known for his deep love and loyalty towards his friends. Fanfiction that has Naruto hating his friends is probably a deviation from the traditional understanding of the character. It might be used to explore some extreme scenarios, like if he was under some sort of mind control or if there was a huge misunderstanding. But in the mainstream Naruto - themed fanfiction, this is not a common occurrence.
In this fanfiction scenario, Naruto and his friends would likely be extremely confused by Cell. Naruto might immediately jump into action, ready to fight. His friends, like Sasuke, would probably assess the situation carefully. Maybe Sakura would be a bit scared at first but then gather her courage. They would have to quickly figure out Cell's abilities, which are very different from the foes they usually face in their own ninja world.
Naruto's power might also be affected. If he was betrayed, he could lose the will to fight or train. He might even start to doubt his own purpose. This could lead to a decline in his abilities, which in turn could put the village at risk if he was its main protector.