If it's someone I'm close to, we have a relationship in which to talk about it and air my grievances, and I probably even want to do that, because by having that kind of conversation, we reinforce our relationship. If, on the other hand, it's someone I merely know socially, for example, I probably don't want to put that kind of energy and work into our relationship, especially if they've just done something that makes me cranky. Why would I want to open up enough to explain what was upsetting to me if that will strengthen a relationship that's not important to me and in which my current feelings are primarily negative?
This usually comes up in the context of acquaintances who I see occasionally in social contexts, and the main reason to bring up any grievance in this setting is to smooth those interactions in the future and possibly to help me not resent seeing them at parties and events. OR in the case of someone who's more socially proximate than that, to avoid the awkward move of ceasing to invite them to social events that I might have invited them to in the past before the toe-stepping.
It seems to me that this is where the interfering social busybody role is important in a larger social circle. I could let drop to ISB that I'm pissed off about how so-and-so did thus-and-such and didn't even have the good grace to apologize, and ISB can take S-A-S out for drinks and say, "Oh, by the way, I'm sure you were already thinking about this and just haven't gotten around to it, but you might want to apologize for that thing ..."