I could be all wrong here but IMO hitting a sore spot simply "one ups" the hitting ya'll do to one another. Again, I managed to get what I thought was a clear thought out all cloudy.
Here is how I perceived it. Let me use Sam and Dave.
Sam suspects that Dave is sensitive about his baldness and so wants to take every opportunity to tease him about it; Dave knows that Sam is going to do that.
When Dave says, "Sam, the next time you mention my thinning hair, I will end the conversation," Sam knows he's got ahold of something juicy. This only matters, of course, if Sam is the kind of person who -- like STBXMRSSP -- likes to have these little things in his arsenal.
If, on the other hand, Dave says, "Well, at least I save wear-and-tear on combs -- maybe I could lend them to you for your back hair," Sam knows that he's going to get as good as he gives and so there's no more incentive to pursue that course of action because Dave isn't going to respond the way Sam wants and, more importantly, Sam is exposing himself to the kind of thing he wants to put on Dave if he continues.
