Why forgiveness is so difficult

We find that the work of forgiveness has to be done over and over again. We forgive, we mortify our resentment; a week later some chain of thought carries us back to the original offense and we discover the old resentment blazing away as if nothing had been done about it at all. We need to forgive our brother seventy times seven not only for 490 offenses but for one offense.
— CS Lewis, Reflections on the Psalms, 25