I have failed to realize that Christian growth is a constant battle against the forces of Satan in my own heart. The battle is spiritual in nature and is fought against my own vicious and destructive thoughts, emotions, and immature ungodly actions. In the Old Testament Jonah was a bitter man. I do well to be angry, even unto death. Here Jonah believes he had a right to his anger.
People enjoy holding things against other people.
But the Ephesians text requires us to remove all bitterness and to maintain a tender heart. These are inner attitudes. Tenderness, be definition, involves a tender loving heart. Bitterness, although inner, is also on the outside, but it is not possible to have two different, contradictory attitudes on the inside. Paul says to get rid of all bitterness and to be kind and compassionate one to another.