I don’t know why.
There are some people you look at whose mouths are just constantly turned down, or their brows are furrowed. They look angry and miserable, and slightly evil. There was a boy at school like that, his mouth was all pouty and like an upside down U … so like a lowercase n … *cough* I can’t say for certain whether he was always miserable, as he was a yob and I don’t associate with those types, but I never saw him smiling. Even when he and his friends had amused themselves by being particularly disruptive, his mouth didn’t turn up.
I am not one of those people, however. My mouth isn’t upturned and smiley, but it isn’t downturned and miserable. I just look serious all the time. It’s my default expression, and I find it rather rude that complete strangers (it’s ALWAYS complete strangers) find it necessary to comment on it.
People look sad/annoyed for a variety of reasons. They just look like it. They’re having a really bad day. They just discovered they’re going to be fired. They ran over a cat on the way to work. Someone stole their lunch that they’d been particularly looking forward to. Drawing attention to any of these things will not make the sad looking person feel better, and will in fact make them feel worse, because their comment makes it seem like your face is offending them and you should do something about it.
My normal response to these people is to nervously laugh, and say I’m quite happy, really. But I think next time I might tell them I’m having my foot amputated the next day. Then limp out of sight.