Consider this. The sense of smell is based on nerve-endings picking up data from airborne particles. This means that when you can smell something, you have already breathed in particles from whatever it is that you smell, which became airborne and eventually reached your nose.
So, when you sniff someone's armpit, you're breathing in a part of the person?:
If only it were that pleasant. We all know that smelly pits is a generalization based on sweating, since the armpits are one of a few natural crevices most prone to heat, and thusly most prone to sweat.
So I'm breathing in a person's sweat?? Ew!:
Less pleasant still! Sweat itself, in fact, has no notable odor. However, you know those little bacteria that are everywhere? Natural things, they exist to break down things.. Biodegrading is not just because things slowly fade away. It's because bacteria eat it. They digest particles, and secrete a more natural substance, which most forms of vegetation can thrive off of.
Bacteria also like to eat sweat. In fact, they'll eat just about anything.. some things just take longer to digest. What you are in fact smelling from your friends' armpits, is the natural substance the bacteria secrete after digesting the sweat. That's right, you are sniffing fecal matter.
So think about that the next time you plan to sniff someone's armpit.. You are breathing in microscopic poop! Nyah!