Carol gave Julia a pink t-shirt that has a picture of a bee and then the word "happy": Be Happy. Julia loves it and constantly asks for her 'be happy' shirt. If one of us says, "Don't Worry," we've taught her to respond with "Be Happy!" And, if she sees anyone who is sad (or pretending to be sad to get her attention...not that I've ever done that), she'll go up to them, pat them on the hand, and say, "Be happy, Bubs (or whoever it is)!"
Don't worry, little girl. It's hard not to be happy with you around.