I have a cat we got from the ASPCA a few years ago, and he's always been very strange for a cat. But I've been noticing that he'll lick his reflection on any glass surface. Why does he do this?
Very few animals understand that a reflection is themselves, they think it's another animal. It could be that he's trying to groom his "friend".
If there is dew/moisture on the glass he could be getting water from it.
He may just like the texture of it, some cats will lick plastic wrappers for this reason.