There are many reasons why we can't forgive another. Often it isn't because of the wrong doing sometimes it may be a case of pride preventing our lips from forming the words, "I'm sorry."
Based on a personal experience and Matthew 18:21-22: "Then Peter came to Jesus and asked, 'Lord, how many times shall I forgive my brother or sister who sins against me? Up to seven times?' Jesus answered, 'I tell you, not seven times, but seventy-seven times.'"