'He forgets nothing but he forgives everything' – in that case, he will be doubly hated, for he makes doubly ashamed – with his memory and his magnanimity.
From Age of Propaganda, by Anthony Pratkanis and Elliot Aronson:
Forgiveness of guilt and compliance were recently investigated in an experiment conducted by Brad Kelln and John Ellard. In their research, college students were led to believe that they had mishandled some scientific equipment and thereby ruined the experimenter's study...But here is the interesting twist. One group of students was forgiven their supposed misdeed. They were told by the experimenter: "Don't worry about it. That's OK."
What would you do in such a situation? Often the act of forgiveness is seen as "wiping the slate clean" - the transgressor is absolved of guilt and the person offering forgiveness is perceived as a friend. However, that is not what Kelln and Ellard found. In fact, they found that just the opposite occurred. The offer of forgiveness served as a double whammy; first they felt guilty about damaging the equipment and then they were offered no means of making it up to the researcher. The only way to make restitution and to show that they were "good" people was to comply with the experimenter's request to do more work. And that they did, offering to do almost twice the work as the other students in the research. But all of these guilty feelings had a cost. When the students were forgiven their transgressions, they came to dislike the experimenter - the person who had absolved them of their crime. Apparently, people do not like people to whom they feel beholden.
This also calls to mind Jonathan Sacks' comment about how "nobody will ever forgive the Jews for the Holocaust."
If it is a deeply ingrained part of human nature to resent people to whom you owe something, I think it's interesting to contemplate what this means with regards to Christianity and its overweening emphasis on the need to be forgiven for your transgressions by someone who's perfect. In fact, I wonder if Jesus is who Nietzsche had in mind with that aphorism.