The only thing I can say is, it's a dogs instinct and licking is there way of looking at the world. Since dogs don't have very good eyesight they have other senses that are stronger, and this happens to be one of them. Much luck!
Sincerely,
BayLee's Mamma Sarah
P.s. BayLee likes to lick the windows in my truck, it's really gross and hard to see out of!