In a letter from the third Lubavitcher Rebbe, Rabbi Menachem Mendel of Lubavitch, he responds to a chossid who was having difficulty being joyful. The gist of the answer is that an individual has the jurisdiction to control his thought, speech and action. These are the main faculties of man and are therefore under his control.
Consequently, a person must be careful to guard his thoughts—to think only happy thoughts; his speech—to be careful not to think about depressing thoughts; and his actions—to act in a way as if he is completely happy. He should move in a joyous way (even if he as of yet does not feel joy), and then, the joy will eventually be true.
With joy, it is essentially true that you can fake it, until you make it!
(Igros Kodesh, Tzemach Tzedek, Pg. 363)