Why does your poop smell bad?

Simply because it's mainly composed of un-digested food. Given that the human body is the ideal temperature for bacteria to grow - your faeces is laden with live bacteria. They have already started decomposing any left-over food in your digestive system - creating gasses such as methane and sulphur dioxide (rotten eggs) - which creates the smell. Additionally, we are conditioned to dislike anything that doesn't smell nice - in case we try to eat something rotten. !