Learning! Beliefs are shaped by actions.
"The things you do often create the things you believe."
Benjamin Franklin said this, in a very astute reflection on what psychology would later confirm. We like to think that our beliefs drive our actions, but we also explain our actions by determining that they must be consistent with our beliefs. Over time, as our actions pile up, so does our understanding of the kind of person we are and how we see the world. Same goes for how we see others.