This seems to be a fairly common scenario. Since I don't work in a public environment anymore, it's been something that hasn't crossed my mind in a long while.
However, today while paying for a few items at the gas station the guy working the register decided to strike up a conversation with me that went something like this:
Guy: "Having a bad day?"
Guy: "Well then, why don't you smile for me? You look mad."
Personally, I find it annoying at best to feel like a stranger is telling me how to present myself. I end up feeling like my personal "bubble" has been somewhat invaded. It's not anyone's choice but my own if I choose to share a physical cue of (emotional) warmth with them and being asked to do so makes me immediately disinterested in everything about the person in question.
However, I really don't know what to say next when people put me in this position. I don't want to "lash out" because I feel that often the people doing this may not understand why it can be inappropriate, but there is absolutely no way that I am going to smile for them, let alone continue a conversation with them.
Given this information, what's the best way to respond to such comments? Is there an "exit strategy" I can take that can express that the reason I am doing so is because of that comment, without coming off as aggressive?
Edit: Here's an article speaking to why this scenario can be off-putting when coming from a stranger, for those who aren't sure where my annoyance is coming from.