Because you didn't serve the L-rd your G-d with joyfulness and with gladness of heart, for the abundance of all things, therefore shall you serve your enemies (Deut. 28:47-48)
How great is the importance of happiness in our Divine service! From the wording of the text we understand that the punishment of "serving your enemies" comes not because of a deficiency in observance or any actual defect in service, but simply because of a lack of joy. Indeed, the emotion of joy is so powerful that it produces a reciprocal response from Above, causing G-d to nullify strict judgment. (The Alter Rebbe, Rabbi Shneur Zalman)
Reprinted from the Parashat Ki Tavo 5761/2001 edition of L’Chaim.