Guys, why do you get cocky if a girl likes you?

It gives the ego a boost in self confidence. Making us more daring and social. Sometimes it only last for a bit, or sometimes we can learn it completely.

Confidence booster. Girls do the same thing too.

And who's to say it's not the other way around, and maybe you just like the guys that are cocky to begin with?

same reason girls do.
its a huge boost of confidence
the more confident some one gets the cocky-er they become.
no every one gets cocky with confidence though.

When we do find out that a girl likes us for whatever reason we become empowered because you have recognized us or something about us and that makes us feel.."special." Some guys may take it too far and boast about it but that's just the select few.

That statement is too general because not all guys do it. SOME GUYS do it because they have an inflated ego by it. That's not necessarily a bad thing but it can be. It makes them feel good about themselves so we choose to express it in different ways based on our personality.