In Matthew 18, we see a picture of the unthankful servant who was forgiven of a great debt, but could not forgive his fellow servant of a much smaller debt. If a person is bitter or hold things against others, then it shows us that they are unthankful for what God has done for them.
Matthew 18:32-35, “Then his lord, after that he had called him, said unto him, O thou wicked servant, I forgave thee all that debt, because thou desiredst me: Shouldest not thou also have had compassion on thy fellow servant, even as I had pity on thee? And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him. So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses.”
If we don’t want to be called “O thou wicked servant” on judgment day, then it is a 100% necessity that we forgive those who have offended us. The Bible is even clear that if we don’t love one another (and bitterness is proof of hatred), then we are abiding in spiritual death:
1 John 3:14, “…He that loveth not his brother abideth in death.”