When I get asked repeatedly "are you okay?" when nothing it outwardly wrong it sets something off in me. "No buddy, I just have perma-bitchface. I don't walk around smiling like an idiot!" Also, anyone telling me: "smile!" I'm an excellent customer service-woman, but a male customer told me that once and I asked him politely (and with a smile on my face): "am I giving off that much negativity?" For whatever reason this did not upset him, in fact came back the next day to apologize. He's a regular customer and still remembers me as the cashier that gave him what for, in a very friendly way of course.
I guess there's something to be said for honesty with customers, especially when it gets them to see you as a human being and not just the robot that makes their lunch....but that's a whole other rant.
P.S. when you type "bitch face" in to Google Images search "Megan Fox" comes up as a related search. Don't laugh at that.