For how long is it reasonable to keep a grudge?

Grudges are not rational at all. They are emotive.The only purpose to them is probably an evolutionary bias to discourage dishonest behavior by making the penalty for such behavior so severe that being dishonest isn't worth it.Keeping a grudge is basically sending

Does everybody lie? Why?

I doubt very much if there has ever been anyone who can claim to have never told a lie and in spite of the fact that it really is not that hard to understand how dangerous and destructive, including self-destructive, lying can be, it is rather sobering to be aware that we mostly choose