My dog does exactly the same thing. Every chance he gets he lies down and stares at himself in the mirror. I have one in my bedroom that's visible from a particular spot on my bed. He always chooses that one spot. I've always wondered what's going on in his mind when he does that. I almost get the feeling he's thinking "Wow, so this is what I look like!". He's incredibly bright, very social, with a lot of personality. Terrier breed, 2 years old.