Dogs are highly intelligent creatures, and just like humans, they have emotional needs. They just aren’t able to communicate verbally, so they instinctively ask for what they need in various other ways.
Your doggie likes the taste of your skin the same way they would want to lick a salty bone or anything interesting tasting, for that matter.
Bear in mind that dogs use their, sometimes overbearing but necessary, licking and sniffing to navigate their environment. You are a crucial part of their environment, so it’s only fair that they investigate you thoroughly.
Our beloved tail-waggers are often licking our faces because of an instinct passed on in their DNA.
A dog licking your face, or any part of your body for that matter, can also be part of grooming. So if you don’t mind the slobber then enjoy the TLC from them.