Why are some people so unforgiving?

I heard a person say on TV that forgiving is a process and I agree. I don't think there is someone not capable of forgiving; it just takes them longer to forgive than the others. We are governed by our biases, prides and prejudices and that is what I think hastens and/or prolongs the forgiving process. If you already apologized for something, then you've already done your part (so long as it's sincere), the rest is up to the other party... :)