I believe spicy foods kill certain types of gut parasites.

So at some point in our evolutionary history some weirdo who liked a bit of spice was born, burned the parasites out of their gut, and then with their increased health and no calories being stolen natural selection did the rest.

Us humans are just a pawn in the chili plants' game of spreading all over the world.

Initially, capsaicin was produced to prevent mammals from eating the fruit (peppers) and instead encourage birds to spread seeds over a wider area (which is why they are brightly coloured to attract birds). Now that humans can travel greater distances than birds, the plants have switched strategies (or at least we've deliberately grown flavourful varieties everywhere we can).

> Several alternative hypotheses were considered-that spices provide macronutrients, disguise the taste and smell of spoiled foods, or increase perspiration and thus evaporative cooling; it also is conceivable that spice use provides no benefits. However, none of these four alternatives was well supported by our data. The proximate reason spices are used obviously is to enhance food palatability.

> But the ultimate reason is most likely that spices help cleanse foods of pathogens and thereby contribute to the health, longevity and reproductive success of people who find their flavors enjoyable.

From: https://www.journals.uchicago.edu/doi/pdf/10.1086/420058

This reminds me of the kind of things one could find on /r/showerthoughts

Same vein: why do we do towel laundry since we're using them after the shower once we're clean.

Why did they become popular recently? Pain causes the release of endorphins, overall making you feel good. A similar concept as a runners high, except achievable watching TV on the couch.

Their original spread was likely for the same reason as most other spices, it could be used as a preservative by masking the taste of semi-rotten food.

> Why did spicy foods become so popular when the first sensation they cause is pain?

Could be related to why BDSM is popular.

You could say the same about cactuses.

Maybe lot of people have dull taste buds and, with pain, they can actually feel something.