External behavior that expresses enthusiasm is the result of an inner feeling of enthusiasm. But it is also true that by behaving externally as if you were enthusiastic, this will also develop a similar inner feeling. If someone is doing a good deed with external signs of speed and enthusiasm, then little by little that outward behavior will influence inner feelings. On the other hand, if someone's external behavior is lazy or depressed, his inner feelings will soon be consistent with that.
This is one of the most important principles for mastery of one's emotional state. Identify the mental states that you want and then act externally as if you were already in that state. This includes your total being: posture, facial expression, and even your tone of voice.
This method will give you increased joy, enthusiasm, confidence and serenity. With practice, you will gain true self-mastery.
(see Rabbi Moshe Chaim Luzzatto - Path of the Just, ch.7)