I read somewhere, that as a youngster she had bad teeth and that made her self-conscious about her smile and it's now a habit not to smile in public.
She's supposed to have a wicked sense of humour though.
I would imagine most people would get sick and tired of the photographers in your face every time you step out from somewhere.
Where ever she goes she is met by all the snapping of the cameras and I think that's probably why her little girl is hardly ever shown to smile 'cos she's probably fed up with it too.
I remember seeing a documentary about VB a few years ago...she was funny as hell. It changed my opinion of her,and I'd not be surprised if she smiles to herslf,and looks glum to deliberately rile the media and paparazzi.