Those who profess to follow Christ have a real and true claim on the forgiveness of their fellow believers. A Christian who refuses to forgive a brother in Christ has made the suffering, death, and Resurrection of Christ null and void for his fellow believer.
The problem is that if I make the passion of our Lord null and void for you, it is null and void, period. Its effects are no longer available to me.
That is why Christ said that if you do not forgive your brother from your heart, neither will your Father in heaven forgive you.
And again, the fifth Beatitude says blessed are the merciful for they shall be shown mercy.