Because they can and because your skin tastes like salt. My dog always tries to lick all the lotion of my mom after she puts it on cause they think it tastes good. It's a part of dogness. I've heard where in the wild, they lick the mother's mouth so that they throw up, but I've never heard that they do that for people, I mean, they might, but, I always thought they were doing it in a loving way, because they know you'll feed them later, they don't need to beg for you to throw up food.