The salt on your skin. Because she enjoys it and it gives her pleasure. Because it's allowed. There could be many reasons. A friend of mine has a dog that we say is a tongue with a dog attached because she loves to lick everyone so much. Her other dog is just about the opposite and rarely licks anyone.
Licking you can be an anxiety/obsession type thing. They are doing that to comfort themselves and produce endorphins. Also dogs will do this as a sign of "i'm lower on the pecking order than you and i know that" BUT in both situations you as the human can make both situations worse (allowing anxiety to get worse, and lower your status) by allowing it to go on. If you do not mind your dog licking you, then one or two licks before you say firmly "aaht!" and block their next move. if your dog still doesnt get it, leave.
Licking themselves obsessively can either be anxiety again or more commonly a symptom of allergy or adopy. Eliminating common allergens like wheat, soy and corn is a great first step to reducing the licking. Also using products like comfort zone can help your dog on a pheromone/chemical level to deal with the issues.
*I should also add if you are talking about a boxer.... that's just the way they are