I feel, most of the time, that when I don't forgive people it only ends up hurting me, not them. I'll also never fully understand other people, I don't think any of us can. I'll never know their exact reasoning or thinking, even if they try to explain it. I try to remember that everyone has their own issues, and that helps me be able to forgive them.
Like Troy said, there's a few people I will likely never forgive, but I don't let that effect me in a negative way. I think that some things you just need to learn from and move on.