system
Given the following post, respond as if you're commenting on Reddit:
human
Why does having diarrhea cause pain in the intestines? 

I understand why constipation is painful, but why is diarrhea painful?
gpt
Diarrhea usually is caused by irritation to the intestinal lining. Your intestinal lining has pain receptors but your brain has difficulty localizing it specifically. The casing around the intestines (peritoneum) has pain receptors that your brain can localize. Irritation causes inflammation, which involves the release of chemical messengers that tend to have a spill-over effect. Inflammed intestines lead to irritated peritoneum. Intestinal lumen pain is a deep dull pain that is difficult to specifically point to as coming from "here."  Instead, the answer will be more like "all up in this general area."
Interesting corollary: Pain from appendicitis develops as the appendix becomes more and more inflammed. The appendix itself does not have awesome pain receptors, but its inflammation irritates the peritoneum.  This causes a dull diffuse pain, mainly around the umbilicus.  As the appendix gets worse, the pain will actually localize to a place called McBurney's point, the point where the appendix physically comes in contact with the peritoneum.  This is usually in the right lower abdominal quadrant for most people, 1/3 of the distance from the anterior superior illiac spine to the navel.