Why should i smile to rude customer?

i am a assist manger in a fast food joint I'm am a kind, mellow, and gentle person who treat everyone with respect and work really hard dealing with rude customer and trying to make sure everyone order is right and proper. My staff were always happy and very well manner we give 110%. The restaurant get very busy everyday and very stressful. But I always respond making sure the customers are happy and got everything they need but there are customer that can be very mean and not patient. I had a situation were a doctor was waiting in start yelling saying what's taking so long then complain about the price so I try to explain that i have no control over the price he refuse to listen he just paid and wait for his number to be called. A second later he came back to the counter and complain where his food. So I told my one of my employees to skip a few orders and do his so he can come down and stop making my employees nerves. We gave him his food and ask if he needed any condiment he said no and sat down then few minutes later he came back complain the food was cold so I apologize to him let him keep the food he bought and went to the kitchen and gave him a fresher food. He took it sat down and ate it then come back said it was under cook and complain about service and said we all need to smile more. The next day got a call from the head top saying we gave a customer lazy service and need to smile more. After thinking about for a while why should i smile to a guy like that who keep me waiting in a waiting room for 3 hr just for a quick check up and I don't be rude nor mean I be nice and very happy to be there. There are days Im stress and don't have the energy to smile especially bad customer I know it part of customer service but sometime there are those days