The substance in chilies that makes them spicy is called capsaicin. It is concentrated in the veins of the fruit (not the seeds) and stimulates the nerve endings in your mouth, fooling your brain into thinking you're in pain. The brain responds by releasing substances called endorphins, which are similar in structure to morphine. A mild euphoria results, and chilies can be mildly addictive because of this hot pepper "high".
The hottest peppers I have experienced, other than premium hababeros are certain Thai peppers that a Thai lady grows near here on her 3 acre farm. She grows exclusively thai produce and makes a pretty good living with it. People come here from a hundred miles away just to buy her peppers at the local farmers' market. She sold them on line for a while but decided she preferred to do her business face to face.