I think it might have something to do with the texture & how it feels on their tongues. Our Muffin will lick pillowcases & towels for no apparent reason. Her favorite thing, however, is my husband's chin & neck when he hasn't shaved. That's where I got the idea that it might be the feel of what she's licking as opposed to the actual object. Then again, I could just be jealous of her spending all her time & attention on someone else!
C.J.