In some cultures, small children are weaned off breast-feeding using chiles: the mums will rub chile on their breasts, the kids will stop drinking (the text I wrote about that fails to mention that it doesn't sound particularly enjoyable for the mums, but anyways...). And still, such children, growing up in cultures using chiles like that, may grow up to eat fire...

Considering such oddities, it's a rather burning question just why chile peppers tend to get popular with humans.

Competing - or Complementary? - Theories

Many theories are put forth by botanists or nutritional scientists. Their background is in natural sciences, and the most popular explanations have some background in evolutionary biology, arguing that chile peppers became popular because of...

• Microbial-antibacterial Effects
  Chillis kill off germs. This is particularly important in tropical regions and when consuming meats. Hence, the popularity of the chiles in those areas.
• Nutritional Considerations
  Chillis are rich in vitamins A and C. These, in turn, are lacking in some traditional diets, particularly such which mainly consume rice. Adding peppers to rice, one gets a fairly decent meal already

There are also suggestions based on physiological and psychological effects of the chile peppers:

• hot temperatures and the cooling effect of chile-induced sweating
• release of endorphins
• (masochism) (okay, that's not considered a valid reason, but who knows...)

It only pushes back the question a little, but there could have been an effect of culture/cuisine:

• Cultural Pre-adaptation
  which is to say that many cultures which rapidly integrated chile peppers into their cuisines already used other hot spices (such as pepper, sansho/sichuan pepper, waterpepper)

One interesting problem with many theories is the modern, rapidly increasing popularity of chile peppers and the pungency they provide. They are becoming so popular, so quickly, evolutionary or "cultural-preadaptive" reasonings don't appear to fit.