Why are people cocky?

Answer 1

Cockiness most likely comes from old, old brain traits to enhance the chances of finding a mate, thus enhancing the likelihood of matings which produce young, thus increasing the chances of survival of the species.

You can see attraction behavior in selection of clothes: either attractive to a potential mate, or comfortable, implying stability and succes, which of course are important features for nurturing the family unit, thus increasing the chances of survival of the species.

Although this behavior may come from 'way back in history when having children literally meant survival, in today's society the behavior is not so important. But the old, old brain traits are still there and active.