Shame and guilt i suppose
Absolutely mate thats the main reason because Shane and Guilt boosting our Ego and Pride not to admit our mistakes and with that we usually deny the fact that we already knew we are in fault
Because of Pride, each party doesn't want to lose on an argument that's why is it hard to say sorry.
But what is pride if we are really the one who commit mistakes?i guess this must be stop if we want to have a happy and live in friendly world