Because we were required to meet those expectations and accept what we were told. It was a matter of survival to us when we were children, and we internalized it because we had no choice back then. Adverse experiences like abuse, trauma, and including emotional abuse and neglect cause damage to our brains that is observable on a PET scan. I can tell people facts about it, but I haven't been able to get past feeling like I am "less than" others, and my confidence and sense of self efficacy were destroyed. For some people, acting in a different way - acting as if you believe in yourself - might repair the damage. It takes encouragement and internalizing new, more positive messages about yourself. You could also use affirmations. I haven't been able to do it that way, and I might get to try EMDR soon. I have a little more hope that maybe EMDR will change something.