I understand that the body reacts to capsaicin in unusual ways. It is an irritant to most animals, hence capsicum spray, and anyone who has ever touched their eyes or ugly bits after handling even a mild one will appreciate this. It is remarkable that the substance is somewhat addictive since while we shouldn't care to repeat uncomfortable sensations, we do anyway.
The 2 Davids :
When I was at college we had an "ornamental" chilli plant that would cover itself in tiny little red fruits that were actually incredibly hot. A couple in a dish, even without the seeds, would spike the whole thing right up, yet we had a friend (David) who could just pick them and eat them straight. Not just one but he had no need to stop.
I have another friend (David) who used to deliberately order his Mexican food and Pizzas with extra heat, so much so that the sweat would run from his brow and drip off the end of his nose as he ate them.
I find it incredible that some chillies require the wearing of protective gloves to prepare a dish, yet there is somebody capable of eating it.
A month ago I was given a box of chilli chocolates for Father's Day by my daughter. It had 12 chocolates being 3 of each in Mild, Warm, Hot and Extreme. The mild was not much more than chocolate. The warm you could feel the chilli but the taste was good. The hot was as much as I wanted to try and I only ate 1. Needles to say the 3 Extreme chocolates await some fool stupid or drunk enough to put it in their mouth.