I hate it too.
I come from a family of people who suffer from resting bitch face. Even our family doctor commented on it to my mother (who married in so looks more cheerful) when I was a child.
We can't help it. We're actually quite happy, it's just hard to tell from looking at us.
When my Grandad died, my cousin had to come on the train to the funeral, she was collected by another cousin at the station.
As she walked through the station, dressed in black, looking generally sad, carrying a wreath, some idiot got in her face saying "cheer up love, you look like you're going to a fucking funeral or something."
To his credit, when she replied with "that's exactly where I am going" he did mutter sorry before he slunk off. I don't know how much more of a bigger clue he needed than a woman in black carrying a wreath.
She was upset at the time but did manage a laugh about it later.
I did post on here though about a rude Mum at school who never speaks to me usually, finally deciding to speak to me in the corner shop by demanding to know why I always look so miserable.
I did tell her I couldn't help it as I have resting bitch face, unlike her who has the more active sort of bitch face that she obviously puts real effort into.
I got to walk away and leave her speechless, with the man behind the counter having a laugh to himself about it. 