I think it is - at least for some people. Saying "I'm sorry" means admitting that you did or said something wrong. Why is it so hard, sometimes, to admit to that?
Obviously someone who doesn't like you or doesn't care will not say "I'm sorry". But, when someone who cares - like your own sibling, or spouse - has such a hard time saying it and chooses instead to wait it out, quietly, hoping it will go away... well, what is it, then? Have you ever experienced this? Finding it so hard to say "I am sorry" - even if in your heart of hearts you know you were wrong - that it makes you angry?