Discrimination

Discrimination is a cruel, but important teacher

For example, if you have a slight resting bitch face, and walk through a grocery store, you will have a hundred micro-interactions where people have to make a quick decision as to whether you're nice, happy, sad, angry, etc. If they get it wrong, and treat you in a way that's inappropriate to your true expression, then it will lower your fitness. In this way, there is significant pressure for our outward expressions to match our inwards, and all thanks to discrimination.