Example:
Your best friend A caught you (B) stealing their money and talking bad behind their back.
A forgives you, giving you a 2nd chance. You learned and never did it again.
Years later, it's A that stole money from you. Same offense you got forgiven for. So now you have to forgive right? It's only fair.
Same thing if cheating happens. If your partner forgave then you're suppose to be forgiving too if they do the same?