I have 2 Cockers and my girl was the same way, I tried everything to get her to stop, but the only thing that worked for me was to give her a little flick on the nose when she was licking, not hard, just enough to puzzle her. Now she doesn't lick people or anything I don't want her to. You have to be consistant. As for a reason, don't know, but could be, habit, nervous, submissive, or obsesive behavior??