Lack of Simchah Gives Power to the Enemy

Rav Mendel taught that lack of joy gives power to the enemy nations, allowing them to rule over us. He read the verse in Devarim (28:47), "Because you did not serve Hashem your G-d in joy, with a glad heart, and you served your enemies," as one statement: because you did not serve Hashem with simchah, you will serve your enemies. If a person lacks joy and delight in fulfilling the mitzvos, this is what gives the enemy nations the upper hand and the rule over him.