system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: why when you have diarrhea your body excretes liquids when you have to go, wouldn't it be better to just preserve the liquids than excrete it?
gpt
The liquidy stuff that comes out when you have diarrhea is what the contents of your digestive tract look like after your stomach, liver, and small intestine add their digestive juices to the mix. Normally your large intestine at the end of the tract reabsorb all liquid which is what makes your poop a soft solid. 

When you have diarrhea, your digestive tract goes into purge mode and expels its contents without waiting for reabsorption. It's a reaction to toxins or infection, so it's generally better to sacrifice the fluid and potential nutrition in order to eliminate a greater threat.