Why do humans seem to be the only mammals that like spicy food?
I think like many things spicy food became something that we used as a tool. Used mostly for preserving food, those countries where it gets very hot due to a tropical climate you'll find the food tends to be much spicer in areas where parasitic flies and other pathogens would contaminate the food faster. The spicier it is, the longer your food stocks would last. Over generations they developed a higher and higher tolerance.