If you deny others forgiveness, you are denying the presence of the Kingdom of God. Forgiving one another is a sign of the kingdom, and when we do not practice it, or when we practice it begrudgingly, we are expressing distrust in the King and disbelief in His kingdom.

From N.T. Wright's The Lord and His Prayer:
[H]aving received God's forgiveness themselves, they were to practice it amongst themselves. Not to do so would mean they hadn't grasped what was going on. As soon as someone in one of these Jesus-cells refused to forgive a fellow-member, he or she was saying, in effect, "I don't really believe the Kingdom has arrived. I don't think the Forgiveness of Sins has actually occurred." Failure to forgive one another wasn't a matter of failing to live up to a new bit of moral teaching. It was cutting off the branch you were sitting on. The only reason for being Kingdom-people, for being Jesus' people, was that the forgiveness of sins was happening; so if you didn't live forgiveness, you were denying the very basis of your own new existence.

The two clauses "forgive us our debts as we forgive our debtors" and "thy kingdom come" in the Lord's Prayer are more connected than we realize.

In previous blogospheric discussions on forgiveness, I have had people express the need for the other's repentance before forgiveness can be properly given. I think there is some truth to that, but I fear making my forgiveness of someone so conditional. It smacks of gracelessness. My God saved me "while I was yet a sinner" -- indeed, because of my own personal theology, I don't believe I could have repented of my sins until God first freed me from them. So just as a personal stance, I would think myself an arrogant grace-denier if I denied forgiveness to those who have sinned against me until their level of contrition pleased me.

I want to forgive not because the forgiven "earns" it, but because the kingdom necessitates it.