Love this. It reminds me of this. See #44.
My default response when exhorted to smile by some random stranger is "why?" It usually shuts them up - mostly because they don't expect any response other than an idiotic smile.
The expectation that women will smile all the time fits with the broader expectation that women will express all kinds of emotions they don't really feel. See for example the form that women's sexual desire is generally expressed in mainstream porn - in a totally inauthentic and male-centric way.
I think that gets at the heart of my issue with this societal expectation that women will smile all the time. If you don't feel like smiling, but you smile to please someone else, isn't that a lie? Feels like a lie to me. So maybe that's the key - being honest about your thoughts and emotions is a privilege that comes with power. If you're unhappy about something that someone did, or you're preoccupied with other things or focused on your current project, you're unencumbered by the requirement of pandering to the emotions of others - if and only if you're in the more powerful position.