"Joy is not an express mitzvah but is the greatest of mitzvot"

"A cardinal principal in the service of God is that it must be done with joy.

It is said in the name of many Hasidic masters that there is something that is not listed in the Torah as a sin yet is worse than any sin and something that is technically not a mitzvah yet is greater than all mitzvot.

Nowhere does the Torah expressly forbid sadness and depression, yet this is the most virulent of sins, for it stifles the heart and mind, closing them to the service of God.

Joy is not an express mitzvah but is the greatest of mitzvot, for it opens a person's heart and mind, enabling him to perform all the mitzvot and to make a mitzvah of everything."

--Rabbi Adin Steinsaltz

From Opening the Tanya by Rabbi Adin Steinsaltz