Joy, teaches the Torah, is to be a perpetual presence in the life of the Jew. There are, however, several degrees of joy.
1) “Serve G-d with joy” (Psalms 100:2). This is the joy that accompanies the performance of every mitzvah—the Jew’s joy in having merited to fulfill the divine will. Here, the joy is not an objective in itself, but a component of another aim: the objective is to serve G-d, but in order that this be achieved in the most optimal manner, one’s deeds must be saturated with joy. (For example: giving charity grudgingly aids the recipient materially, but also demoralizes him; giving cheerfully nurtures the pauper’s body and refreshes his soul).
2) “Seasons for rejoicing” (Kiddush for the Festivals; Deuteronomy 16:14). On the festivals, it is a mitzvah to rejoice. Here, joy is not an accompaniment to some other deed, but the substance of the endeavor itself. Nevertheless, this is still not joy for the sake of joy. The objective remains the fulfillment of the will of G-d, who commanded that the festivals be celebrated joyously.
3) “When [the month of] Adar commences, one increases in joy” (Talmud, Taanit 29a). This means that the Jew strives to increase his joy in all areas, including his non mitzvah-related activities. The joy of Adar is not a joy with an objective, but an end in it itself.
4) “A person is obligated to get drunk on Purim until he doesn’t know…” (Talmud, Megillah 7b). On Purim the Jew attains the ultimate in joy: not only is his joy not qualified by any reason or objective, it is free even of the objective to be joyous. He is so consumed with joy that he is oblivious to all, including the fact that he is rejoicing…
From an address by the Rebbe, Purim 5718 (1958).
. Likkutei Sichot, vol. IV, p. 1274.