It's funny, my great grandparents had a golden retriever that did this. If she came over and you didn't pet her she would sit there and lick your leg with her big sloppy tongue until you did. She also licked tables, furniture, and feet. It was kind of weird but they never tried to make her stop. I wonder if it's an oral fixation because she also had a bad habit of chewing on rocks, which really hurt her teeth. Maybe try getting enticing new toys/chew toys that the dog would want to lick and chew on instead? Good luck 
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