The Sages say that the Shechinah does not rest on someone who is sad, but only on someone joyful in doing the mitzvah. I tell you, a person can achieve what the greatest rabbis and mystics of former times attained, but sadness will prevent it. Therefore, you should always be joyful, especially when engaged in divine service, for without joy one cannot cleave to the Holy One, blessed be He.

Baal Shem Tov , The Light and Fire of the Baal Shem Tov by Yitzhak Buxbaum