Have you ever worked in a shop, OP?
It's tiring, can be very boring and monotonous, you have to deal with some downright sods of customers, have to work every weekend and bank holiday, and get paid a pittance. It's very easy to be nice to people who are nice to you, but much more difficult to be friendly and chirpy when someone is just plain rude. Are you always friendly to the shop workers? It cuts both ways, you know.