That's one of the things it's called. Poop, shit, feces. Informal, impolite, and more formal.

[She can't help but crack a small smile at his tone when he says 'poop,' even if the conversation, as a whole, just prompts a brakes-skidding sound in the back of her mind.]

A lot? But it's because poop is gross and dealing with it is bothersome. People will also be weirded out by a request for samples because what people do---and produce---in the bathroom is generally nobody's business but theirs.