See to it that no one comes short of the grace of God; that no root of bitterness springing up causes trouble, and by it many be defiled; Hebrews 12;15 (NASB)
Rejection is such a deeply personal hurt because often it is not our deeds or actions that are rejected, it is us as people, our very person-hood is what seems to disqualify us from being a part of the life of that person. There is little we can do when someone finds our very existence unacceptable to them or too distasteful for them to be around.
The inborn response of the heart when a person is rejected is to become angry at the one who has hurt them so deeply. Typically, the hurt and rejection are replayed over and over in the mind and the person comes to actually cherish the hurt and the anger goes deeper into the heart. It is from that anger that the disastrous fruit of bitterness is born.
The more the rejection is replayed the more justified a person believes they are to be resentful and angry; the more they thing they have a right to take revenge in some way, and they have a right to be bitter!
Scripture tells us that bitterness and wrath is what results when the pain of rejection is not dealt with biblically. I seriously doubt that a person who has been rejected understands this is the end of the road of withholding forgiveness, but it is.
The wrath of the wounded heart determines that someone is going to pay for how they have been hurt, and that bitter wrath destroys all possibility of a future relationship. Sin is made manifest in a variety of ways: they may do a lot of yelling and hollering, or they determine that everyone around them must hear of what has been done to them, and every kind of evil speaking flow like a river from their heart.
Bitter people often become poisonous, they infect everyone around them over time. Everything is negative, everything is awful, everyone is terrible. They are complainers and short tempered and simply not nice to be around. In the end, they become the very thing they hated.