Belly buttons smell because they generally don't get paid much attention to in the shower. It is a dark, deep place that collects sweat and produces bacteria. This makes them smell bad when not cleaned regularly. Clean your belly button! Look here for more information: http://www.wikihow.com/Clean-Your-Belly-Button
Belly buttons smell when they have not been cleaned or when they are infected. Your belly button is very susceptible to dirt and it's very important that you don't forget to clean it a couple of times a day.