We all mirror each other constantly. Think of a time when you were in a good mood and encountered someone, at work possibly, who was in a lousy temper. What happened? Chances are that your mood shifted. Maybe a lot and you became grumpy or maybe just a little and you felt a tinge of irritation for the rest of the day. Perhaps it went the other way and your pleasant mood rubbed off onto the grouch.
This is called "The Mirror Effect" and is part of our natural clairvoyant ability to read other people's mood. Sure sometimes it is obvious, but other times it is subtle and wordless. We may not even be aware that we are picking up on someone else's mood and making slight changes in ourself to accommodate for that.
Pay attention to it next time it starts to happen. When you see Grumpy coming, recognize it immediately and in this way you will become less inclined to mirror that bad mood. In fact, you can become the agent for change as you set your intention to be the brightness that others mirror. It could make the day go a whole lot smoother.