I try to smile.
But mostly to cover up my horribly bad avoidant personality disorder and gradual mental deteriotation, but also because I look like a badger with a slight monobrow stuck in a barbed wire fence and that I'm going to murder your family when I'm doing anything but smiling. Seriously, I don't even have a monobrow.
I also have grey eyes, so I look dead inside, which, combined with my borderline tics and bitchy neutral face, make me look unaproachable, unsympathetic, and when I smile, it, like, gets worse, because it pushes my cheeks up higher than other peoples', so you can add "manic" to that pile too.
So, my resting face looks like Renee Zellweger, except slightly less dead inside.