Facial expressions are contagious

The book I'm reading says about facial expressions.
They are contagious because facial expressions are unconsciously mimicked. This facial mimicry sends signals to the brain, creating the emotion that is insistent with the facial change.
It's said like this;

" happy faces evoked increase activity in the zygomatic muscles involved in smiling, and angry faces energized the corrugator muscle used in frowning. Furthermore, happy faces increase the subjects' feelings of happiness, and angry faces increase their feelings of fear. Fearful faces too, such as contraction of the corrugator muscle of the forehead."

I think it means that you should keep smile even if your neighbors are upset, you should be cool yourself not to be affected by the mirror but you can send your smile back instead.