Is there any worse confidence killer than rejection? I think it goes back to childhood, when you want a new bike for your birthday but you end up getting a pack of tube socks instead. You immediately wonder if you did something wrong and that's why you didn't get what you wanted.
Go forward a few years when you end up taking your cousin to the prom because everyone else turned you down. And the college years? Basically a parade of rejection that feels like an endless line of Rockette kicks to your confidence.
Or maybe that was just my experience.