Joy Keeps Us From Serving Our Enemies
The Rymanover read a verse in Devarim as one statement: lack of joy in mitzvos is what gives power to those who oppress us.
The Torah says in Devarim (28:47) that we will suffer because we did not serve Hashem in joy and with a glad heart, and we will serve our enemies. Rav Mendel read the verse as one continuous thought: failing to serve Hashem with gladness is itself what leads to serving enemies. When a person performs mitzvos without delight, without feeling the privilege of doing them, this very lack of joy hands strength to the forces that rule over him. In other words, joy is not a luxury added to avodas Hashem. It is what keeps a Jew free and standing on his own ground.
Source: Menachem Tzion, as cited under Yeshaya 58:13, on Devarim 28:47
Choose one mitzvah you perform every day, such as davening, a bracha or giving tzedakah, and do it today with deliberate gladness. Before you begin, take one breath and remind yourself that you are being invited to speak with the King. Notice if your posture, your voice or your pace changes. If joy does not come, it is enough to act as if it will. A real smile, even a small one, is itself a step toward freedom from whatever is weighing you down.