"And when you stand praying, if you hold anything against anyone, forgive him, so that your Father in heaven may forgive you your sins." (Matthew 11:25)
When someone does something that causes us pain, it is natural to nurse a grudge against him (or her). But the lesson of the Gospel is that we are called to forgive just as God forgives us. In fact, verses like this one may lead us to think we can only receive as much forgiveness as we extend.
When forgiveness is genuine, it is a beautiful, spiritual thing. Mark Twain once said -