In the human world before civilization, there were moral norms, but not law. In that world, people were taught how they should ideally enforce moral rules. If B harms A in violation of local moral norms, and if A confronting B gives an inadequate response, then A should tell near associates about it, and this group should talk and coordinate, deciding who to believe and what to do about it.
However, in practice the folks around A and B faced a strategic choice. Even if they believed that B had harmed A, as A had claimed, they’d still have to wonder if it was in their interest to take A’s side or B’s. If B were more dominant, prestigious, or valuable, or more successful at threatening retaliation, then it might be in most of this group’s interest to favor B.
If A could anticipate this response, A might not bother to complain about B to associates. And anticipating this, B might feel more free to harm A in this way. Or A might at least expect this. At which point A might well resent B.
If A instead expected support from the near group, their reaction to B’s harm might be outrage, indignation, long-suffering, or perhaps even indulgence and forgiveness. But it would not be resentment. A would be instead be energized and optimistic regarding the response to their complaint. But A is instead resentful when the group has not supported them, or when expecting that they didn’t bother to complain. A might even try to illicitly retaliate against B in secret.
Thus in this world, resentment is a bad sign about a potential associate A. They feel they’ve been wronged, but had either insufficient evidence or social support to successfully prosecute it. Which suggests that they may be biased about B, or have low local status or value. If you become A’s associate, A may well push you to support their weak complaints that others won’t support. They might even ask you to look away from or join in on their secret illicit retaliations.
In fact, A might appeal to egalitarian norms to gain your association - what, aren’t they good enough for you? And then you might reasonably resent their putting you into such an awkward spot, especially if pointing to their resentment as your reason for rejecting them opened you to accusations of not enforcing moral norms.
Our modern world adds law to this situation, but law is expensive, doesn’t apply to all harms, and often favors those high in status. Thus even today resentment remains a bad sign about a person. It is a sign that they feel they’ve been wronged, but others don’t support this claim.
Note that this resentment effect adds to B’s initial harm to A. Not only is A harmed in the initial manner, but A is also harmed by being made to resent B.
Note also that if A’s early teachers had expected them to be low status, they might have reasonably taught them fewer moral norms. The benefit of a norm helping to coordinate behavior might be less than the costs of resentment when others fail to support A’s complaints based on the norm. Thus those low in status and value might reasonably embrace fewer norms.
Added 19Dec: According to the above analysis, we should be more accepting of resentment when it is done because of wrongs done to others, not ourselves, when the wrong is too small to justify an immediate overt response, and when the resolution process is expected to take a long time, and the current time is early in that process.
The inspiration for this post was someone telling me that I seem resentful, and they didn’t like that.
More evidence that Robin is our Nietzsche.
Off topic, but scenarios are more parseable if you call the characters human names like Alice and Bob instead of A and B.