Retracted my previous comment, because it was agreeing with your claim that it's better to never hold grudges in the first place, which I quickly realized I also disagreed with.

A grudge is an act of retaliation against someone who has harmed you. They hurt you, so you now retract your cooperation - or even engage in active harm against them - until they have made sufficient amends. If they hurt you by accident or it was something minor, then yes, probably better not to hold a grudge. But if they did something sufficiently bad, then it is better to hold a grudge to show them that you will not accept such behavior, and that you will only engage in further cooperation once they have made some sign of being trustworthy. Otherwise you are encouraging them to do it again, since you've shown that they can do it with impunity - and by this you are also harming others, by not punishing untrustworthy people and making it more profitable to be untrustworthy. You do not forgive DefectBot, nor do you avoid developing a grudge in the first place, you hold a grudge against it and will no longer cooperate.

In this context, "forgiveness is a good thing" can be seen as a heuristic that encourages us to err on the side of punishing leniently, because too eager punishment will end up alienating people who would've otherwise been allies, because we tend to overestimate the chance of somebody having done a bad thing on purpose, because holding grudges is psychologically costly, or for some other reason.