Why do we, as a collective, have such a mive tendency to feel 'not good enough'?

It's the route of so much of our unhealthy behaviour - from girls not being able to leave the house without 10 layers of make-up on, to overreacting partners feeling insecure about their other half going out for some beers. It all seems to come down to a fear that we don't deserve the best and therefore do/will not have it.