Everyone always says that forgiveness a worthwhile life strategy, and is for you, not for the other person who wronged you. This seems obviously true in some cases–in principle if you are nursing a rather trivial grudge which is bothering you, it would be better to let it go. In severe cases there is evidence that anger or misery can dampen your immune system, shave years off your life, give you heart disease, etc. The NYT has recently advocated both the somewhat paradoxical advice to hold on to grudges under certain circumstances, and the more traditional suggestion to let go of them. (At the former link there is a kind of fun quiz you can take to see how serious the grudge is, and whether you should allow yourself the petty pleasure of nursing it. Also, it’s clearly meant to apply to that girl in fourth grade who said that you used crayons and colored pencils on your poster of the solar system, and it didn’t match, and she didn’t want to sit with you at lunch for three days.) The latter is the advice most often given by psychologists and 12-step programs and self-help books.