When you were hyper as a kid and basically useless to the monk who was supposed to oversee your upbringing so you literally did stupid shit to get attention...

It's how he learned that he was good at the sword. But he hadn't the aptitude for much else (magic, basically) and was considered a bad fit for a guardian trainee or warrior monk. He was too gung-ho... and asked too many questions. He questioned a lot about Sin and Yevon... and when it started to affect the students alongside him, to the point they started asking questions as well, he was kicked out. He's basically been alone since.

Even though the Djose Highroad and temple fill him with mixed feelings, he visits often, because it's on the way to the Moonflow, which has always been his favorite place in all of Spira. He's spent many days running away from everything to be along its banks... and he's worked odd jobs for the Hypello, which is why he loves the Shoopuf so much.