It's an old virtue that's thought to be good for society, which I imagine is why it's proselytized. But refraining from offering forgiveness need not be a burden. There are people I will never forgive, nor bother to think about. It's not a matter of carrying hatred, it's just a judgment. Forgiveness is an absolution that returns people into good graces: I don't see why, in a blanket sense, everyone who's done exceptional harm should return to everyone's good graces. Petty slights can be more easily forgiven, as everyone will eventually cause offense and consider it a mistake.
