I had a lady who I used to work with stumble upon my Facebook profile at some point and when she saw the (limited) photos of me she reached out to me and told me that I should smile more at work. It's actually pretty rare the number of people who get to see me smile these days.
I find those people (at work or otherwise) who stick their faces in other people's and say, "smile," to be annoying and selfish cunts who have no idea what a person might be going through internally or in their life that might cause them not to be smiling at that given point. It's rude, selfish and ignorant. A much better way to do it perhaps would be to (genuinely) ask, "are you ok?" and then (genuinely) care about the response.
Yeah, that is definitely a better way to approach it I think. I usually do have a lot going on in my head at any given moment and I am usually just trying to keep it all straight, so being sure I am smiling is not at the top of mind for me!