I've worked as a waitress and bartender for a few years now, and it always angers me when customers demand that I smile at them. Why do they care if I'm smiling as long as I give them their food and booze on time? What's so wrong with a snarl here or there?
I hate it even more when my waiter or bartender gives me a fake smile. It makes me feel like they hate me and are about to spit in my food.