Dogs pant to cool off when they are feeling hot. When our body temperature gets too high, we sweat. Evaporation of sweat from the skin takes up a lot of heat, and helps cool the body. But dogs are almost completely covered with hair, and have every few sweat glands.
They can’t keep cool by sweating. Instead, a dog pants. It sticks out its tongue and breathes rapidly. By panting, a dog moves air back and forth over the moist tissues of its mouth and tongue. As the moisture evaporates, body heat is carried away, and the dog feels cooler.–Dick Rogers