The chemical responsible for the heat in chili peppers is called capsaicin. It activates pain receptors in our mouths, tricks our bodies into thinking that we're hotter than we are, and even increases our metabolism. This in turn triggers the body's cooling mechanisms, making us sweat and become flushed.
In effect, capsaicin is a drug! A relatively mild and short-lived one, to be sure, but it does affect the normal functioning of our bodies. And like a drug, our the strength of our reaction depends on how much we eat, how quickly we eat it, and how tolerant we are of it to begin with.
As we eat spicy foods, we end up ingesting more of that sneaky capcaicin, and that's why the heat seems to build. More and more pain receptors in our mouths become activated with each bite. We sweat more profusely and get red in the face. We can even become light-headed and euphoric! Our bodies feel the affects more strongly and sometimes for longer.
Some of us really like the sensations that come from eating really spicy foods. Others, not so much! Where do you stand?