Why do dogs do this?!

My second rescue dog, Charlie, is a really neat dog.... So is my first dog, Rocky, but he constantly tells me he's a real boy....

When Rocky was a puppy he used to lick me, the days, however, he just licks my spot on the bed so that I continually find myself going to bed in a massive pool of dog lick.

Charlie, however, licks whoever is closest, and licks,and licks at like a hundred times a minute - often faster. As a matter of fact, he has earned the nickname "Whiskey" Charlie because he is such a licker.

Why?