Ever wondered why we humans love spices? Isn't it strange that despite so much of burning sensation, humans still like to consume spices? But before that, let's try to find out why some plants produce spices like chillies, black peppers, etc.
One of the biggest threats for most plants is the proliferation of fungi and various pathogens which literally destroys plants (fungi attacks) within a matter of hours or days. This is more prominent in tropical climate where the temperatures are suitable for microbial activity which in turn spoils the plants. So over time, some plants started producing hot & spicy "fruits" & "herbs", which have anti-microbial properties, as a form of self defence to protect themselves from such pathogens. In fact, the plan worked out well, as the spicy components of the plants could prevent all forms of microbial activities and thus protect itself. Also, due to such pungent odour, it could keep various animals at bay, thereby protecting the plants from animal invasions as well.
Coming to humans, as we all know, there are tons of microbial activities happening within the human body. There are good microbes as well as bad microbes, but thanks to the body's immunity system, bad bacterial activities are suppressed. But how does the human immunity system suppress such microbial activity? This is where we must revisit the above mentioned phenomenon. Just like how plants realized that usage of organic compounds with spicy taste and pungent odor can prevent microbial activity, similarly, human body had also realized (as part of evolution) that spice and pungency can prevent pathogens.
But unlike the plant, the human body didn't have an easy mechanism to produce it. Hence, humans started depending on such spice plants so that the same spices which were used by the plant for self defence, could in turn work for humans as well for self defence. But there was a small problem. If such spices are consumed excessively, then they might ruin the whole digestive system of humans and lead to other complex problems. The key was to consume spices, but in moderation. But how will the body know what is moderate level?
That's where the sense organs had to be rewired, so as to resist excessive intake of spices, but at the same time, still find pleasure in spices when in moderation. As a result, the tongue was designed to cause burning sensation, while the nose and eyes were designed to become watery, whenever spices were consumed above the body's tolerance limits. That was the body's way of signaling to stop overeating the spices. But when consumed in moderation, the same sense organs would trigger pleasure, activating the senses to higher levels and so on.
All these are body's way of motivating the human to eat spice so that it receives it's regular dosage of anti-microbial chemicals which in turn suppresses the pathogens and boosts body's immunity. So, this is the whole story of how plants initially developed spices for themselves as self-defence which was later "plagiarised" by humans for the same purpose of self defence and immunity, by rewiring their entire human sensory system to feel motivated (i.e. enjoy) to consume spices, but in moderation.