A smile can boost your emotional state because when you smile your brain releases neuropeptides that aid in fighting off stress. Also, neurotransmitters like dopamine, Serotonin and endorphins are also released which further leads to increased mood.
It is mentioned in the text, “inhibiting facial reactions can dampen your reactions to emotional images” (Rosenblum, 2010, p. 191). Meaning that when you restrain from facial reactions you actually bring your mood down.
In the book there was an experiment conducted, This experiment used a pen and it showed that when an individual held a pen between their teeth, their mouth was serendipitously put into a smile; so in return, the experimenters found that when they looked at the cartoons they were amused by versus when the individuals held the pen with their lips, their mouth was put into a frown and the experimenters found that the individuals found the cartoons less amusing.