What makes a person "feel bad" about themselves?

Since the beginning of the self esteem movement, people have been admonished to “feel good” about themselves.
I don’t really understand the description-I think that it should be “feel effective” but that is another question.
So exactly why would a person “feel bad” about themselves? Outside of disability or disease, most people control their actions and the consequences are in general pretty predictable. So why would a person perpetually “feel bad” about themselves when, within reason, they pretty much have control over how they live out their lives?
BTW, I am sure that most folks “feel good” about themselves (I do).
And I usually can find something redeeming about anybody and everybody.
So, what I really want to know is why somebody would actually “feel bad” about themselves?