There are two premises.
First, scientific research has demonstrated that when we look at other people's facial expressions, we tend to imitate them in an imperceptible and subconscious manner through a process known as facial mimicry. The hypothesis is that this behaviour helps us understand the emotions we are watching.
The second thing we need to know is that our expressions, just like our face, are asymmetrical: they are more pronounced and begin earlier (facial expression is a dynamic process) on one half of the face compared to the other. This asymmetry is thought to give expressions a special "flavour": those that begin and are more pronounced on the left, for example, are judged as being more authentic.