Why does natural gas smell like poop?

They actually put an obnoxious smell into it so you will notice a gas leak before it kills you.

In order to assist in detecting leaks, a minute amount of odorant is added to the otherwise colorless and almost odorless gas used by consumers. The odor has been compared to the smell of rotten eggs, due to the added tert-Butylthiol (t-butyl mercaptan). This smell is similar to that you find when you visit sulphur springs such as the one in Rotorua, New Zealand -- and is generally offensive to most people.

So the poop smell is similar to that of rotten eggs.

natural gas doesn't really have a smell on it's own. They add the poop smell.

Its "mercaptan" a chemical that's added to the gas so people can smell the leak.