But after having a brisk (ie. as yet unthorough) run through the text, I have to wonder: is it happiness that is most contagious, or is it the physical manifestations of happiness, which may not be actual happiness itself?
(I’m presuming that “happiness” here is the same as “deep contentedness” – ie. lasting and profound, not quick, fickle and fragile).

Being around people is a great way to get a lift of mood. We’re social animals and we’re biologically encoded to dig that. And being around happy people makes us respond that way. But sometimes it’s “crowd happiness” – we become part of the Big Happy Vibe. It’s a social convention, and we want to all fit in, so we act happy – whether or not we really are…