He's not like the others. When Xerneas finds out his father isn't even from his world, he seeks to find out who, and what he is, as well as who his father was.

A glimpse at what he later learns as Equestria, has him learning about the powers of friendship and inspires a new sense of purpose with his newfound abilities.

When he returns to a troubled home-world, he has to try and resolve things at home, while trying to figure out how to get back to finish what he started. But the Equestria he left could not prepare him for the one he returns to.