Cats like to smell their owners’ armpits and also like to lick them, because this is related to the secretions and smell of our armpits. Cats are firstly fascinated by its smell, and secondly, they analyze its "information" before they smell/lick it.

Because they like the taste and smell of humans. Cats have special olfactory organs on their tongues that help them recognize and remember tastes. For cats, licking a human's armpits may be a way of showing affection and intimacy, as they associate the smell of humans with feelings of safety and home.