I found this Gizmodo article, which I found amusing since it was something that was recently brought up.

http://gizmodo.com/5645331/why-do-humans-love-spicy-self+torture

http://www.nytimes.com/2010/09/21/science/21peppers.html?_r=1

However, after recently being naive to a great deal of wasabi in my soy sauce , I tend to believe that the joy of spice is reserved for people prepared for it.