Poo certainly is very smelly. Some poo smells worse than others. Poo smells due to the work of bacteria in your tummy. The bacteria change your food into various compounds and gasses that make it smell like poo. If you eat lots of meat, or only meat then your poo will smell really bad. Animals that only eat meat have very smelly poo.
The bacteria in your tummy make a smelly gas called Hydrogen Sulfide. On its own it smells like rotten eggs. A mixture of things make your poo smell the way it does.
One of the chemicals in poo that make it smell is called Skatole. This chemical is also used in some perfumes and even as a flavouring in ice cream! Yuk!