Resentments have a tendency to accumulate. They are something that grows. Bitterness is a settled negative disposition toward someone, but it is a settled negative disposition that feeds and grows.
If you one time fired an employee for laziness or dishonesty, that might represent a settled negative disposition toward him, in that you would never hire him again. But that is not bitterness, but rather simple wisdom. However, if you have a negative disposition toward someone that seizes eagerly on new information about that person, information that reflects badly on him, then that is bitterness.
“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” (Heb. 12:15).
Bitterness is described here as a root. It is underground, and what it does down there is gather up nutrients. That’s what roots do. Those nutrients are supplied by various talkers and gossips, or perhaps Facebook updates, and so the store of negativity grows. Bitterness can grow on negative reports that happen to be true or false. But the longer the process continues, the less necessary it actually is that the reports be true. After the roots grow to a certain size, the lies are in fact preferable—sort of the devil’s Miracle-Gro.
Notice also that roots have a tendency to come to the surface. The writer here describes them as “springing up.” Roots lead to fruit, and the fruit here is corruption and defilement. When a bitter root springs up, when the nastiness goes public, the end result is that many are defiled.
The last point to realize about this is that bitterness has a tendency to look away from itself to the real or imagined offense of the other. If you tell a lie, you cannot think about the situation without thinking of your own lie. If you lose your temper, to think about it at all reveals your own offense to you. But when you are bitter, every time you think of the situation, your roots are eager to do so.