Maybe he's been through some tough experiences like losing a friend or facing a great failure, so he needs time alone to heal. It could also be that he's discovered some dark secret about himself or his world and is trying to come to terms with it in solitude.
It could also be due to a sense of guilt. If something bad happened that he blames himself for, like someone getting hurt because of him, he might exile himself as a form of self - punishment. He'd rather be alone and deal with his own pain than face those he feels he's let down.
Maybe he feels guilty. Suppose he made a big mistake, like accidentally leading an enemy to a safe haven of demigods. The guilt could be so overwhelming that he exiles himself to punish himself and keep others safe from his potential future blunders.
One possible reason could be jealousy among his peers. Maybe someone was envious of Percy's skills or popularity and spread false rumors about him, leading to his betrayal. For example, if Percy had a special power or was favored by a certain group, others might feel left out and decide to turn against him.
One reason could be the presence of more powerful or seemingly more capable characters around him. He might constantly compare himself to them and feel inferior.
In such a fanfiction, one possible reason could be his feeling of extreme isolation. After his estrangement from his family, he might have felt completely alone in the wizarding world. He was always so focused on rules and order, and when things didn't go as he planned, especially in his career and family relationships, it could have pushed him to this dark place.
It could be that Percy's sense of honor led him to sacrifice himself for Zoe. If he had made a promise to protect her, he would uphold it no matter what. Also, Zoe might have been the key to solving a major mystery or threat. Percy, realizing this, chose to sacrifice himself so that Zoe could continue and complete whatever task was necessary. Additionally, his love for his friends and the greater community of demigods might have made him believe that Zoe's survival was more important than his own.
It could be related to his past traumas. All the battles he's been through, the losses he's suffered, might be weighing on him and he doesn't know how else to deal with the emotional pain.
Perhaps there is a misunderstanding. In the fanfiction, other characters might have misinterpreted Peter's actions or words, and this has led to an uncomfortable situation for him. As a result, he chooses to distance himself to avoid further conflict.
One possible reason could be the burden of his past. Naruto has faced a lot of hardships and the memories might be overwhelming him, making him want to be alone. For example, the loss of his master Jiraiya might have left a deep scar, and he might feel that isolating himself is the only way to deal with that pain.
Maybe he feels a sense of guilt. For example, if something bad happened in the village that he blames himself for, even if it wasn't really his fault. He could exile himself as a form of self - punishment. He has always been very hard on himself when it comes to the well - being of the village and his friends.