Boy, Anno, you are so right-on!!!
I remember once when EW pointed out that people thought she wasn't happy, or was possibly mad (it's been awhile - not remembering exactly how she put it) because she had a naturally down-turned mouth. I have the same and know exactly what she spoke of. Back when I worked (in accounting), I'd be walking down or up a hall and someone would pass and see me thinking hard about something. They made mistakes, thinking I was upset, when I wasn't. The expression was a natural resting/contemplative expression - when I think, I think hard... get wrapped up in my thoughts, powerfully.

We all have moods, all of us. As for me, I don't think I show my moods that much (I wear a perpetually blah mood - it keeps me from getting hurt), so it'd take someone that really and truly knows me to interpret a mood of mine skillfully.... I'm speaking for myself here.
I dare say, most people don't have the talent (or sage-like skills) it takes to adequately understand another person's true mood, unless they live like moods. I know people who could bluff their way through the holocaust. I know people who practice not showing their moods. I know people who want their moods to be recognized... and so on.