Why does Paul tell the Corinthians to forgive the man they had disciplined?
Paul directs the Corinthians to forgive and comfort the disciplined man so he will not be swallowed up by excessive sorrow. Since the majority's punishment had already dealt with him, the church needed to confirm its love toward him and restore him rather than leave him in despair.
Les explains that in I Corinthians 5 the church had taken corporate action; the majority voted to deal with the man because of gross immorality, though a minority wanted to leave him alone. Then in II Corinthians 2:6-7, Paul says the punishment inflicted by the majority was sufficient. Therefore, on the other hand, they ought rather to forgive him, lest he be swallowed up with overmuch sorrow, and confirm their love toward him. Les points out this is not the attitude most people would take, but it is what believers are supposed to do even today.
So that contrariwise ye ought rather to forgive him, and comfort him, lest perhaps such a one should be swallowed up with overmuch sorrow.