The best way to not be disappointed is to first realize that everyone is different, second that no one is perfect, and third is to forgive. If just gets easier from there.
Who cares? Who died and made them Lord of all? And what do they do, waste time in revenge or trying to control something they can't? Trust me, I'm not nearly as weak as they are miserable.
Don't trust in them and you will avoid disappointment
Everyone disappoints at some time or another. The best way to reduce disappointments is to keep the ones around who apologize.