Resentment is the persistent feeling that you're being treated unfairly - not getting due respect, appreciation, affection, help, apology, consideration, praise, or reward. It keeps you locked in a devalued state, wherein it is extremely difficult to improve or appreciate or to connect positively with people in general. It carries fantasies of retribution, which stimulate small doses of adrenalin and cortisol for temporary increase in energy and confidence.
Resentment rarely goes away on its own, simply because it doesn't produce enough adrenalin for the amphetamine/crash effect of stronger forms of anger. While exhaustion limits the duration of rage, you can stay resentful for years on end. Without the exhaustion factor, the retaliatory fantasies of resentment persist long enough to become habituated. Thus resentment is more of a mood than an emotional state, and the behaviors it motivates are more habit than choice, with disastrous effects on health and well being.
The habitual nature of resentment means that it is never specific to one behavior - nobody resents just one thing - and that its content is rarely forgotten. Instead, each new incident of perceived unfairness automatically links onto previous ones, eventually forging a heavy chain.
The chain of resentment always extends into the distant past. In advanced stages it goes into the future. That's when you hear things like, "It's going all right now, but she'll find some way to screw up the weekend," or, "It's fine at the moment, but the ‘real him' will come out, just wait."
The Chain of Resentment is Self-linking
The tremendous effort required to drag the chain of resentment through life makes us hyper-vigilant for possible ego offenses, lest they "sneak up" on us. In other words, the chain of resentment makes us look for things to resent. This creates frequent sour moods and an atmosphere wherein no offense is too trivial or too unrealistic to be added as yet another link on the chain. We'll find things to resent in the news, traffic patterns, a dearth of parking places, the temperature of drinking water, and in other people's tastes, thoughts, opinions, mannerisms, and feelings.
A member of a court-ordered class I once taught had a colorful way of describing the effects of resentment. He said that dragging the chain of resentment through life is like carrying around a bag of horse manure. (Okay, he did not say "manure.") You want to smear the bag of horse do-do in the face of the person you resent. So you carry it around, waiting for the opportunity, and carry it around, and carry it around, and carry it around.
And who stinks?