Back in school theatre, us kids were frequently told to "put on your plastic smiles" before going out on stage. I think this is true if you're doing a job, as well. It's like someone is paying you to be the face of their business. But, I agree, when I am home, if I'm sulking, so be it! Bad feelings are still valid.
RE: If you're grumpy and you know it, stomp your feed!