Because they do this and we see it in ourselves and say thank you for showing me.
“Selfishness, self-centeredness! That, we think, is the root of our troubles. Driven by a hundred forms of fear, self-delusion, self-seeking, and self-pity, we step on the toes of our fellows and they retaliate. Sometimes they hurt us, seemingly without provocation, but we invariably find that at some time in the past we have made decisions based on self which later placed us in a position to be hurt. So our troubles, we think, are basically of our own making.” p.62