I think sometimes it's a foible of being human - not wanting to look at and own your own stuff or admit you were wrong. Sometimes it is misunderstanding on one or both sides. I also think some things are easier to apologise and take responsibility for than others - whether consciously or subconsciously.. and then it maybe also depends on how the other party is handling what has happened, how they react, and how that is interpreted... Very complicated, this therapy lark.