Why should I smile when there is no joy in me? The answer is that smiling is a practice.
There are over three hundred muscles in your face. When you are angry or fearful, these muscles tense up. The tension in these muscles creates a feeling of hardness. If you know how to breathe in and produce a smile, however, the tension will disappear —it is what I call “mouth yoga.”
Make smiling an exercise. Just breathe in and smile; the tension will disappear, and you will feel much better.
—Thich Nhat Hanh
Cattāri Brahmavihārā 
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