People tell me I look rather fierce. I think that is because, being smaller than me, they look up at my face from below. Take a mirror and check for yourself. Place the mirror below face level, keeping your mouth as straight as you can, and the image seems to curve down. Put it higher and your lips seem to curl into a smile, although you haven't moved them. This is because the mouth is never straight; it curves back at the sides - 'up' seen from above and 'down' from below.
Evolution has made men on average taller than women. So does this mean that men are more likely to look stern to women than to each other, and women more friendly? Certainly children usually look up at adults and are more likely to see the corners of their mouths as turning down. Perhaps the curled-lip smile, as opposed to the toothy smile, evolved partly to counteract this signal. Perhaps the respect shown to tall people results partly from their apparently stern lips. There's an experiment for someone to do.