I have forgotten most of the pharmacology, but, basically capsaicin and similar alkaloids (capsaicinoids) present in the peppers interacts with a receptor (TRVP1) in your mouth (and the other end, and other places including those which are related to nociception). With enough stimulation, you experience desensitization, i.e., the same amount of capsaicin produces less activation at the receptor than it did previously. Whether this is due to down regulation of the receptors or some other mechanism, I don't remember, and am too lazy to go wade through a ton of literature to figure out. The net effect is that, given an equal amount of capsaicin, folks who eat a lot of hot peppers do not "burn" as much as those that do [not].
The position exists, however, that sufficient desensization allows those folks to experience flavor nuances that mild-to-medium folks, like myself, do not experience (because we are in flames
). It might be difficult to make a compelling argument either way, but they certainly could have a point.
It is interesting because two people could agree that "heat" should not be the main event for the food - yet for a given dish, for one person it is the "main event" and for the other it is not, and the latter person may be getting some flavors that the former person can not get.
...and then, of course, you can take it to ridiculous levels, which serves only to be entertaining to watch on video...and from a safe distance