I figured this out a couple months ago, and it still stumps me as to why this happens. I have 5 cats. A mom, a dad, and their litter of 3 (they're a little over a year). Everytime you scratch one of them right above the tail they go crazy and start licking. Whatever is the closest thing, they'll start licking. Your arm, a chair, a table leg, the garbage can, etc. I have no idea if this is a reflex or something. I've tried this expiriment on other people's cats and they don't lick. Does anybody else have any cats that do this??
It's the same for dogs, perhaps it's an inherited characteristic that has no use but still remains within mammals as a whole.
even people have the "ticklish" spots. My cat doesn't do the whole crazy tongue thing. She does arch her back to be petted by her tail, but nothing like the licking. I hear this is very common, though and I have seen it several times. My cat is fixed. Maybe the urge is stronger in unaltered animals?
My dog does the same thing, same itchy spot too. I asked the vet once and she said that sometimes a flea allergy or the like can cause a spot to be overly itchy, and they'll react that way. Sure 'nuff, when we're careful to keep her bug-free, she doesn't do the licking thing anymore!