Mosiah 26:31

"And ye shall also forgive one another your trespasses; for verily I say unto you, he that forgiveth not his neighbor's trespasses when he says that he repents, the same hath brought himself under condemnation."

Forgiving others is a hard thing to do sometimes. It is easier to remember all the bad things that people have done and to continually hold that against them.  Though easy, this has bad consequences.  God wants us to forgive others when they have repented.  If we don't do this then we have a greater punishment.