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.