Perhaps Percy has received some bad news or a prophecy that makes him think that by ignoring everyone, he can protect them. So he distances himself from his friends and family, and the fanfiction could explore how they react to his sudden cold - shoulder. They might try to figure out what's wrong with him and how to bring him back to his normal self.
Well, perhaps in this fanfiction, Harry has been through so much that he decides to isolate himself from everyone. He might be tired of the constant danger and the expectations placed on him. So he shuts out his friends like Ron and Hermione, and just focuses on his own inner journey, maybe to discover some new magic that he believes can only be found in solitude.
Perhaps Naruto has a new power that he doesn't understand and it's making him act strange. This power could be related to some ancient jutsu. As a result, he starts to ignore everyone because he's afraid of hurting them or because he's constantly in a trance-like state due to the power's influence. His friends would then try to figure out what's wrong with him and how to bring him back to normal.
Percy's family members could be in the fanfiction too. His mom, for example, would be worried sick if Percy started ignoring everyone. She'd be trying to figure out if something happened at school or with his friends. His dad, Poseidon, might also play a role, perhaps sensing Percy's distress from afar and trying to intervene in some way.
Maybe he got some kind of curse. In the Naruto universe, there are all sorts of dark magic and curses. If he was cursed, it could make him withdrawn and cause him to ignore everyone.
One possible plot could be that Percy finds himself in a world where everyone he knew has had their memories wiped about him. He might start by trying to figure out what caused this. He could go to his old friends' places, only to be met with blank stares. Then he might try to find a way to reverse the situation, perhaps by going on a quest to find a magical artifact or meet a powerful being. Along the way, he could face challenges alone, as he has no one to rely on, which would force him to grow and discover new abilities within himself.
Well, it could be that Percy has had some really bad experiences in his life that made him bitter towards everyone. Maybe he was betrayed by friends in the past, so now he just can't bring himself to trust or like anyone.
One possible reason could be that Naruto sees something in Sai that others don't. Maybe Sai has a unique way of understanding Naruto's pain or his dreams. Another reason might be that Sai represents a new beginning for Naruto. Since Sai has a different background compared to the others, Naruto might find his perspective refreshing and be more drawn to him.
The plot might involve Naruto being influenced by some external force. For example, a new enemy could be using some kind of mind - control or illusion on him to make him ignore the Rookie 9. Another possibility is that Naruto has become so focused on his training to become stronger that he starts to neglect his relationships with the Rookie 9, thinking that his power is more important for the future of the village.
One possible plot could be that Naruto is somehow corrupted. Maybe he's been influenced by a powerful dark force or a powerful enemy has tricked him into believing that everyone is a threat to the world he wants to protect. So, he decides to take extreme measures and kill everyone to 'create peace' in his warped sense.
Maybe Harry has had some really bad experiences that made him bitter towards everyone. For example, he could have been betrayed by his friends multiple times.