Our facial expressions have quite an impact. When someone has a wrathful expression, it affects us. Even if it has nothing to do with us, we feel threatened. We take the scowl or frown very personally. Although no words are spoken and we may not even know the scowling person, we may feel deeply threatened, wounded, or scorned. This may be ridiculous, but it happens.
While an ever-smiling countenance may seem a bit extreme, a friendly expression makes people feel more comfortable than scowling, wrathful frowns.
— Pema Chodron, No Time to Lose