The real problem with the sin of unforgiveness is that it is an on-going condition of a person's heart. Let's look at some of the reasons for a person having an unforgiving spirit. Unforgiveness can result from self-love. God commands us to love our neighbors and even our enemies. Love is greater than faith.
Love is greater than hope.
Unforgiveness is proof of a lack of genuine love in your heart. Another reason for harboring unforgiveness is because of having a wounded spirit. Someone else has forsaken you or betrayed you or your spouse has committed adultery against you or otherwise emotionally or physically hurt you. And concerning eternity, it will involve at least the loss of rewards.