Forgiveness is letting a person go.
When you forgive, it is like setting a person free, or better still, setting a bird free from its cage-never to come back again.
No matter how terribly a person might have hurt us, even if we term it "an unforgiveable offence", the person can still be forgiven.
Forgiveness makes you and the other person(s) free. You should forgive because it is an instruction from God our heavenly father to forgive.
You should forgive if you also want to be forgiven.
After forgiving, you should learn to forget-let the bird go forever.
Moreover, why shouldn't you forgive anyway, if Jesus Christ could forgive you of all your misdeeds.