>>20429343Yveltal, the legendary of death, is happy and goofy because it always causes death and thus doesn't get a lot of affection. That, and because it's the legendary of death, it knows how short existence is, and is happy to spend as much time with you as possible before you die.
Xerneas is more or less always aloof because it know, being immortal, that if it were to get attached to you, it would only make your inevitable passing more sad, and it doesn't want to make you immortal just so it can keep you as a friend forever, because then you'll feel the same way when you outlive your family and friends.
