What is it with humans and eating, smoking, or drinking any kind of leaf, seed, or other vegetable that has evolved a deadly toxin to avoid being eaten..?
“You know what, this tastes a bit bland, let's add some insecticide to it to make it spicier!”
We get bored and don't like bland things. These plants developed extreme flavors to deter most animals, but humans like to have unique and different things so it makes sense
I think spicy food is eaten in Spain, in India and in Thailand, because the spice acts as a disinfectant, which removes all kinds of germs (and therefore diseases) from your body, which is especially important in hot and humid climates, because bacteria spread like crazy there?
Also, spices can hide the fact that the food's gone slightly off due to improper preservation (at least until it comes out one end or the other in a ballistic fashion, but then it's possibly too late to pass the blame), so there might have been a bit of that, too.
Still, it's funny how we love eating stuff that'd kill or seriously inconvenience most everything else.