According to this hypothesis, activating the muscles that form facial expressions associated with basic emotions can sometimes create the experience of having an emotion:

I believe the  answer is: Facial feedback hypothesis

Facial feedback hypothesis stated that the facial expressions that we made could actually influence the emotion that we feel.
For examples, studies shows that people who control their expression to constantly smile in all of their situations, tend to be more relax in handling their situations, even if they're under stressful situations.