Forgiveness

Colossians 3:13 (NIV) Bear with each other and forgive whatever grievances you may have against one another. Forgive as the Lord forgave you.

Why must we forgive?
The best reason to forgive is because Jesus commanded us to forgive. We learn from Scripture, if we don't forgive, neither will we be forgiven:

Matthew 6:14-16 (NIV) For if you forgive men when they sin against you, your heavenly Father will also forgive you. But if you do not forgive men their sins, your Father will not forgive your sins.

Forgiveness can only be fully achieved after we have accepted Jesus Christ as our Lord and Savior; otherwise we are simply acting according to our flesh. While we can learn to "forgive and forget" on a carnal level, as a good deed, but true forgiveness comes from the Spirit of Christ within us. Until our mind is renewed, until we are enlightened by His Holy Spirit, there remains a resistance, an enmity between us and God that will govern our every decision
We forgive out of obedience to the Lord. It is a choice, a decision we make. However, as we do this "forgiving," we discover the command is in place for our own good, and we receive the reward of our forgiveness - freedom.