The special rejoicing of Sukkot stems from the spiritual service of Rosh Hashanah, the Ten Days of Repentance, and especially Yom Kippur. As is known, spiritually the sukkah covering (which derives ultimately from the clouds of glory that protected the Children of Israel in the wilderness) is elicited from the repentance of Yom Kippur and the cloud of incense of the High Priest’s Yom Kippur service in the Holy of Holies.

The spiritual service of Yom Kippur, which is “once a year,” stems from yechidah, the attachment of the essence of the soul to G‑d’s essence, “unity to unify You.” On Sukkot this attachment is brought to revelation, until it envelops the whole person, encompassing all his concerns. This is also expressed in the mitzvah of the sukkah, where everything a person does, including eating and drinking and the like, is encompassed by the mitzvah. Therefore Sukkot is the time of the water libation and Simchat Beit Hasho’eivah-a super-rational rejoicing that transcends all limitations-for the joy stems from the revelation of yechidah, the super-rational, unlimited essence of the soul.

(Likkutei Sichot, vol. 24, p. 249)