The seed of bitterness is a hurt that is planted in someone. It may be intentional or unintentional. There are also times when the hurt may be chastisement from God for our own actions. It takes strength to let go of bitterness. The danger in yielding to bitterness and allowing it to rule our hearts is that it is a spirit that refuses reconciliation.
Follow peace with all men, and holiness, without which no man shall see the Lord. Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled. (Hebrews 12:14-15, also Eph. 4: 31-32).