One theory is Poop is funny because it’s one of the five humors, feces, urine, puss, blood, bile. I’ve heard it said that laughter is a direct result of pain, a way to jumpstart the heart. This is why comedy and tragedy are forever intertwined. The five humors are also thought to represent personality character archetypes that can be found in stories from Shakespeare to sitcoms. I’ve heard Winnie the Pooh referred to as a straightforward example of that. Pooh is bile because he’s dumb and happy and hungry all the time, EOr is feces he’s always depressed, Tigger is blood rambunctious outgoing, rabbit is pus “death” irritable and resistant to change, piglet is urine who is always afraid. All people are said to be dominated by one of these five archetypes, that’s why when you see these character archetypes in a group it’s inherently entertaining because it mimics the parts of your own mind. Humans are not the only mammals that laugh, and I like to imagine people laughing at each other for stabbing their toe and farting long before a common language.