Well, we can't be in a good mood all the time. When I was working, usually knew when I was in a "bad" mood. First thing I said to co-workers was "Having a grouchy day. Not you, but me." This caught on with other workers. One lady in particular was on a continued grouch. My first words to her were "can I talk with you today". After a while, she was in a cheerful mood more often. Be up front if this continues, It's ok.