No two farts smell exactly alike, according to this interview with Dr. Lester Gottesman, a proctologist from St. Luke's Roosevelt Hospital in New York City.
The smell has to do with the amount of absorbed products like methane, which is made by fermentation of what we eat, and that's what causes the bad smell, basically. As a baby, when you're born, passing through the vagina, you're infected by the bacteria in your mother's colon, and that's the bacteria you're dealt for your lifetime. Also, everybody is different in how they'll digest wheat products, milk products, whatever. And if they are not digested properly there will be a lot of methane produced and a lot of acid, and that would tend to cause a stinkier bowl movement.