NewMexiKen found this in a 1992 New Yorker article about chiles and New Mexican cuisine.
According to scientists who have studied the effects of fiery food, a very hot chili sends the nervous system into a state of panic, and the brain reacts by flooding the distressed nerve endings with endorphins, which are the body’s natural painkillers—a sort of friendly morphine. The sudden shot of endorphins is what transforms the pang of hot food into pleasure, and also what makes it considerably more tolerable after the first few bites.
The article, by Jane and Michael Stern, is not available online.