When cats lick their humans' armpits, they may be imitating the licking behavior of their mother or companion. From a cat's perspective, a human's armpit is a soft, warm, and sensitive area that closely resembles their hair. Therefore, cats may view the human armpit as another object that can express intimacy and trust.

In addition, cats' licking behavior may also be related to their cleaning habits. Cats are very particular about their cleanliness and will lick their fur with their tongues to remove dust and dirt. When cats lick a human's armpits, they may be trying to help the human "clean" the area, although this may seem unnecessary or uncomfortable to humans.