Mia's like that (though not a lab of course). I think it's just individual personalities. I got a picture of her really grinning today and then realized how unusual that is. She's just a very intense dog. Beau on the other hand is not... he's a permanent goof. ALWAYS has some dumb grin on his face.
When I think of labs, I think of goofy, happy dogs but to be honest, our lab Pete was pretty intense and serious. (He was also field trial bred so I dunno if that had anything to do with it)