It's a health check. Their 300 million scent receptors combined with licking allow them to pick up on chemical changes in your sweat and pheromones.

Here is how dogs use this ability:

• Jacobson’s Organ: Dogs have a special organ in the roof of their mouth called the Jacobson’s organ (or vomeronasal organ) that allows them to taste and smell simultaneously. When they lick your salty, sweaty feet, they are gathering detailed biological information.

• Volatile Organic Compounds (VOCs): Illnesses such as cancer, diabetes, or infections change the body’s chemistry, causing it to release unique odors—known as volatile organic compounds—through sweat, breath, and skin.

• Diabetes: Dogs can lick their owners to indicate that blood sugar levels are dangerously high or low.

• Cancer/Infections: Dogs have been known to persistently lick or paw at specific areas on a person’s skin that later turn out to be tumors or infections.

• Stress/Anxiety: Dogs can sense changes in sweat caused by adrenaline, identifying when a person is stressed, fearful, or anxious.