Resentments and grudges weigh heavy on us. Being unforgiving also has to do with judgement which in itself is a defense mechanism. We may pre-judge how a person reacts, and the negative, unreal situations start playing out in our heads. We tend to forget, that everyone changes, and wants to be a better person. For all we know, they may be holding on to guilt, to how things could have been better if they had not committed that stupidity.