Olfactory Fingerprint

Armpit sweat glands are known as apocrine sweat glands, and instead of producing just sweat, they also produce a mix of proteins and lipids. The bacteria on your skin love that stuff and eat it up, producing waste products in the process. It's those waste products that cause pungent armpit odor.

There’s a reason why we secrete these oils and proteins in the first place. Hair under our arms and between our legs acts as a dry lubricant. When it isn’t enough (during intense exercise or prolonged walking/running), these secretions help reduce the friction and damage to the skin.

Each person has a unique combination of bacteria on their skin which in turn makes them have a unique smell, kind of like an olfactory fingerprint. Humans still use it to recognize family and friends from everybody else on a subconscious level.