There's a reason this is one of my new favorite pictures of Boss. It's because he has lived with me for a little over a year now and him rolling over on his back was something he NEVER did until very recently, and now it's an almost daily occurrence. To me it's a sign that he finally feels comfortable enough to show that vulnerable side and relax.
Lately I have caught him sleeping like this in the middle of the night. It makes me smile, and even tear up a little bit with happiness when I see it.