In a modern revelation the Lord explained:
“My disciples, in days of old, sought occasion against one another and forgave not one another in their hearts; and for this evil they were afflicted and sorely chastened. Wherefore, I say unto you, that ye ought to forgive one another; for he that forgiveth not his brother his trespasses standeth condemned before the Lord; for there remaineth in him the greater sin. I, the Lord, will forgive whom I will forgive, but of you it is required to forgive all men. And ye ought to say in your hearts-let God judge between me and thee, and reward thee according to thy deeds.” (D&C 64:8-11.)