Looking at flavors from the perspective of the organisms that produce them is an interesting exercise.  

To berries, for example, sweetness is a source of energy and a reward they bestow on the animals that spread their seeds. To lactic acid bacteria, sourness is an antimicrobial waste product generated the same way carbon dioxide is exhaled. The rich and deep base notes of a rose? To the plant, it’s a signal to pollinators made by recycling vitamin A.   

Spicy, meanwhile, is a form of protection and something of a cosmic joke. I’ll explain. 

Imagine you’re a pepper plant in the genus Capsicum looking, as all plants do, for a way to defend yourself that doesn’t require running away or moving around in some fashion (since you are, quite literally, rooted in place). If you were a cactus, you’d evolve to develop physical spikes. If you were poison ivy, you’d figure out how to make resin that damages the skin.  

Chiles got creative, combined the two strategies, and created a molecule called capsaicin. It’s a molecule born out of thriftiness: chiles, like all plants, like to reuse and recycle, taking chemicals they produce in great quantities and then trimming and decorating and remixing them into new molecules, rather than starting from scratch. One well-trodden trick is turning amino acids (the building blocks of proteins) into alkaloids like quinine, nicotine, morphine, DMT, and, you guessed it, capsaicin.