Karen has the right idea on this subject. I have just been through a pile of photos and it seems that smiles are rare things, at least in the streets that I have been snapped in.
In this one, I was waiting for whoever took the pic to get on with it, impatient that was me...but then perhaps she was waiting for me to smile. lol. The other variation that I have come up with is that the closer I was to the camera, then the more likely a smile was....
There we have it, street smiling seems rare and personal smiling varies according to the circs of the pic.
I smile tho', when I see this one, taken in the subway, where people do not smile.
~Samm