I don't get the obsession with people wanting her to smile. Just because someone smiles doesn't mean they are happy, the same way if someone isn't smiling it doesn't mean they are unhappy or sad. I don't like smiling either so I don't, doesn't mean I'm not happy or miserable.
Equally for all we know her house could be filled of private pictures with her grinning away. We only see what she wants us to see and I'm sure if she was that bothered what people thought and wanted to appear to be "happy" then she would smile a bit more in public.