There is a fascinating reason dogs seem determined to sniff people in places humans consider private. To us, the behavior can feel awkward or embarrassing, but to a dog, it is simply information gathering. Dogs experience much of the world through scent, using their extraordinarily sensitive noses to recognize people, animals, emotions, and changes in their surroundings. Certain areas of the human body, particularly the groin and armpits, contain apocrine sweat glands that release chemical compounds carrying distinctive odors. When a dog approaches someone and investigates these areas, it is essentially reading a biological message that humans cannot detect. The scent can tell a dog whether someone is familiar or unfamiliar and may reveal changes in hormones, stress, diet, or health. What looks like bad manners is therefore a natural canine greeting and investigation.

The science behind a dog’s nose is even more remarkable than most people realize. Humans have millions of scent receptors, but dogs can have hundreds of millions, depending on the breed. Their brains also dedicate far more processing power to smell than ours do. Dogs can distinguish subtle differences between individuals and remember scent patterns long after a person has left. Their extraordinary ability has made them valuable in search-and-rescue operations and medical detection. But there was one discovery about this ability that made me look at my own dog completely differently—and it happened when he suddenly became obsessed with sniffing one particular person.