One reason we ought to forgive those who've hurt us is simply because God commands us to do so. Jesus warned, "If you do not forgive men their sins, your Father will not forgive your sins" (Matthew 6:15).
But why does God command us to forgive others -- even if they don't deserve it or even want it? One reason is because of what an unforgiving spirit does to us. Think of it a moment: If someone hurts you, what happens to your attitude toward them? You know the answer: You'll be filled with anger and bitterness, and you may even try to strike back and hurt them in some way.
And these feelings will stay in your heart as long as you fail to forgive that person. In fact, they'll become like a poison in your soul, robbing you of your joy and turning you into someone who is sour and bitter and angry at life. The Bible warns, "See to it... that no bitter root grows up to cause trouble and defile many" (Hebrews 12:15).
But it doesn't need to be this way -- not if we begin to see the other person through the eyes of Jesus. He loves them in spite of their failures -- and so should we. Don't let bitterness crowd out your joy, but ask God to help you forgive others just as He has forgiven you -- freely and fully. Then ask Him to help you show Christ's love to them, and to bring them to Himself.