system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does hot and spicy food make your nose run?

As in title. 

NB- I used the search bar beforehand, and despite this having been asked a few times, albeit in quite different ways, most had been ignored/had half-hearted responses.
gpt
Think of mucous like a force field around your weakest places. It'd be bad if there was a hole in your colon that allowed stuff to leak into your body. It would be bad if there was a hole in your stomach that allowed undigested food to get into your body where it shouldn't be. This could kill you. Bacteria are constantly trying to eat you alive. They want to eat whatever organic matter that they can by spilling digestive juices all around themselves and then soaking up the proteins that they break their environment down into. You are that environment.

Mucous acts like a force field by keeping these bacteria and fungi from being able to latch on to your very sensitive parts that allow you to survive (eyes, nose, mouth, esophagus, lungs, stomach, small intestine, and colon.). These parts not only trap bacteria in the mucous, but they also see them quickly out of that body part by moving the mucous so the invading bacteria can't multiply.

When you eat some poisons, your body's natural defense mechanism is to protect iself by ramping up mucous production like crazy to help see whatever that thing is, out of the body. This is why when you eat certain toxic mushrooms (especially the hallucinogenic variety), your nose runs, your eyes get insanely wet/leaky, and you urinate much more frequently. Your body is doing everything it can to get whatever harmful thing it can out of it.

Now, there are three main compounds that cause the sensation of heat from spicy food. One is found in mustards/horseradish/wasabi, and it's called allyl isothiocyanate. The plants have precursor chemicals that produce this oil when they are combined by the action of crushing the plant. This helps protect the plant from being eaten by animals. Capsaicin is found in many peppers, and pipirine is found in pepper drupes used to make black pepper. Again, these spicy chemicals smell and taste bad to mammals, so they protect the plant. It's just that humans are monsters. We like pain. So we actually breed these plants to be even more repellent to us.

Capscaicin and isothiocyanate are powerful irritants and cause inflammation in your body. They stimulate many of the same responses in the mucous membranes as an infection, and the body triggers its mucous forcefield to go into overdrive to get these oily compounds out of the body as fast as humanly possible. This is why spicy food not only makes your nose run, but also has a tendency to trigger nausea, diarrhea, and extreme aversion behavior in most people. It can irritate the lungs on the way down too, causing fits of coughing, which is mechanical motion to repel irritants. It also stimulates the secretion of gastric acid in the stomach, which can cause heartburn. Again, that acid is another attempt for your body to break down harmful invaders to your body. Your body temperature rises in response to spicy food too. Again, this is because your body thinks it is under attack and is trying to kill off an infection that isn't really there.

TL;DR: Your body thinks capsaicin is hurting you, and wants to get it out as fast as possible. The mucous helps get it out of you.  


EDIT: No, just because your body is trying to get it out doesn't mean it's bad for you.