Spicy foods have antifungal and antibacterial properties. In this light, humans have culturally and genetically evolved a preference for spicy foods because they protect us from microscopic assailants. When our taste buds encounter pungency, it's a signal to our brains that the food is cleaner.

Preference for spicy foods, like all foods, is the result of a complex interplay of genes, culture, memory, and personality,  a complex design that scientists are only now beginning to understand