Matthew 6:14 “For if you forgive men their trespasses, your heavenly Father will also forgive you.
15“But if you do not forgive men their trespasses, neither will your Father forgive your trespasses.
Firstly, the power to forgive sin is Gods alone. Here, Jesus enlists us to confirm His will to forgive. We only can forgive because God first forgives and its His desire. Second, our forgiveness is closely linked with our relationship with the Father thus offering that same forgiveness we graciously took to our fellow man. One who doesnt allow that conduit of His forgiveness shouldnt rely on the hidden assumption they have recieved this gloriously freeing forgiveness. Those who know they are forgiven much know they are now empowered by God to forgive their fellow man much. Our spiritual health is dependant on this. This is not an elective as a Follower of Christ. This is required because we are ambassadors of Christ, thus should properly reflect His heart and love.