Unfortunately, part of the service industry, if you’re customer-facing, is looking like you’re having a good time. It requires a mask of false happiness, and it can take an emotional toll on you. You come home at the end of the day feeling like a birthday clown taking off his face paint to reveal a tired, beaten-down old man. The retail smile is a big part of this. This seems to be especially true if you are a woman. Women are expected to be cheerful as a default, and any deviation from that feels like anger or attitude. Men are often not held to the same standard, since we expect them to be serious and taciturn. 

This is obviously a double standard and should be handled tactfully by managers. The best managers understand that you are overworked and underpaid, and being asked to shellac a smile over your face can feel like a demeaning order.