It could also be due to the high expectations placed on him. Everyone expects him to be a great hero, and the pressure of living up to those expectations makes him unsure of whether he can actually do it.
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.
Percy was a character full of self - doubt. In this fanfic, a new threat looms over the land. Percy, despite his uncertainty, is pushed into the front lines. At first, he stumbles, making mistakes. But then, he witnesses the faith that some of the younger heroes have in him. This gives him a newfound determination. He starts to analyze his strengths more objectively. He uses his speed and his ability to think on his feet. By the end of the battle, Percy still has moments of self - doubt, but he has learned to use them as motivation to keep getting better.
One possible reason could be extreme despair. Maybe he faced some overwhelming challenges or betrayals that made him feel there was no way out. For example, if he lost all his friends and felt completely alone in the world of the fanfic.
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.
One reason could be adoption. If a girl is adopted and the adoption agency doesn't have accurate records or if the information is lost over time, she may be unsure of her age. Another reason might be if she was born in a place with poor record - keeping, like a remote village. Also, in cases of displacement due to war or natural disasters, important documents like birth certificates can be lost, leaving the girl without a clear knowledge of her age.
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.
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.
One possible reason could be a misunderstanding. Maybe Annabeth made a decision without consulting Percy, like choosing a quest that she knew was dangerous but didn't tell him until it was too late. Percy might feel left out and mad about it.
One possible reason could be Percy's recklessness. He often jumps into dangerous situations without fully thinking them through, and Annabeth, being the more strategic one, might get frustrated and yell at him for endangering himself and others.