resentment has to do with ones inability to accept a person/place/thing... or situation as it is... ego has a common character defect of projecting it's will... it's ideololgies... it's desires... it's expections... and perspective onto others... and when others fail to live up to/or perform in the way the "ego" expected them to act/perform... then resentment is born...
basically it's as you said rudy... it's an inability to forgive...
which i think stems from an inability to love... think about it... if i truly do love you then just about anything you do will be acceptable to me... i will accept your bad behavior far more readily if i truly love you... "UNCONDITIONAL LOVE"... which i think is the way God loves us... which is love without any demands whatsoever... to love you and accept you just as you are... and it certainly is not easy to love unconditionally... i think only God is capable of unconditional love... we humans are always seeming to be put demands on others... and others seem always to fail us... thus resentments occur... lol... didn't mean to write a book... ;-)