Why do humans stink if they don't take showers?

Answer 1

If you got your hands dirty by touching an unwashed toilet, it will be the pee or what which sticks to your hand.

If you're referring to sweaty armpits or what it'd be the bacteria that thrives in those areas which gives the pong.

Answer 2

It is because showers and baths help to keep the bacteria on the body in balance. It also helps remove chemicals excreted in the skin. Male hormone metabolites tend to give off an odor, and showering removes some of them from the skin. Showering and bathing also reduce the amount of dead skin cells. Odor-causing bacteria can be trapped between the dead skin cells, so removing them helps.

Then there are external odor sources. If you don't change your underwear regularly and clean down there, you will start to smell like urine and feces. Proper bathing removes body waste from the outside of your body, as well as things you step into or splatter on yourself.