Spicy Food Is Not Hot. Your Nervous System Disagrees.
Take a chilli pepper straight from the refrigerator. It is cold. Bite it. Your mouth reports fire.
There is an obvious disagreement here between physics and experience.
The pepper has not suddenly raised the temperature of your tongue enough to burn it. Instead, it contains capsaicin.
Capsaicin binds to a receptor called TRPV1 found on sensory neurons involved in detecting potentially harmful conditions. TRPV1 can respond to genuinely high temperatures. It can also be activated by capsaicin.
So a chilli pepper does something wonderfully dishonest. It presses part of the same biological alarm system used for heat.
The nervous system receives the signal and produces the familiar burning sensation even though your mouth is not being thermally cooked.
This explains several other oddities surrounding spicy food. Your face may flush. You may sweat. Your nose may run. Your body is responding to an apparent thermal emergency created by a plant molecule.
Water does little to rescue you because capsaicin does not dissolve well in water. You can move the burning sensation around your mouth with a heroic glass of water, but you have not necessarily removed much of the compound responsible.
Foods containing fat can help because capsaicin interacts better with fats than with water.
Repeated exposure also changes how people experience chilli. Sensory neurons can become less responsive after repeated capsaicin stimulation, one reason experienced chilli eaters may tolerate concentrations that make somebody else reconsider every decision that led to dinner.
The stranger question is why people enjoy it.
Pain is normally useful because it tells us to stop doing something. Spicy food creates a controlled version of that warning inside a context we know is safe. You get the alarm. You get the physiological response. Nothing is actually on fire.
Humans took a plant's chemical defence mechanism and turned it into seasoning.
As a species, we are difficult to discourage.