Perhaps it's some kind of environmental indoctrination or the result of depression, but I seem far more attuned to the negative energy some people project than whether or not they're happy. Most people I see out and about don't look happy at all, however.
I can accept I often misread people, too. At my old work experience, one of my temporary colleagues told me she was going upstairs to take a private call from her boyfriend, came back down after some time with red, puffy eyes holding a tissue, and I was immediately asked her sympathetically if she was getting a cold. Urgh, it all became obvious afterwards, but I still kick myself over that one.
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On hiatus thanks to someone in real life breaching my privacy here, without my permission! May be back one day. +tips hat+