Art is supposed to make you feel things, even — especially — things you don’t want to feel. Nobody’s ever left a theater and said, “What a great film. I was perfectly comfortable the entire time.”
Often, it’s a (sometimes literally) visceral reaction to being forced to confront the horror of a situation. Other times, it’s a gross-out (again, occasionally literal) gag that pries the laughter of shock out of you. In both cases, it can be so extreme as to bypass the part of your brain that interprets meaning directly to the part that brings up popcorn.
It was largely those instances r/Movies had in mind when it responded to the question, “What scenes in movies gross you out or cause even the slightest of discomfort to watch?” Reddit had no problem remembering the gore, gristle, and plain old cringe that made them heave their Reese’s Pieces.