Percy is an son of Aphrodite, he has known it his whole 6 years of life, his dad told him so, (even told him about the other gods and goddesses)he even heard them talk about how they didnt want him and how they wanted to "try for another kid", whatever that means. He knew why they didnt, and he has to say, he cant wrap his brain around it. Why would his mom and dad not want him, their own child? He knows its because he isnt good looking enough in her eyes but he is still growing and thats pretty rude for a mom to say and his dad just went right akong with it. So he did the one thing he thought was best when neither parent wants you, he decided to run away.