We rejoice on all of the holidays, but Succoth contains an exceptional measure of joy. In fact, this is the most prominent aspect of the holiday. Succoth is identified in the prayers as z'man simchateinu, “the Season of our Joy.”
Why does this holiday of rejoicing immediately follow the High Holidays, Rosh Hashanah and Yom Kippur — a solemn period of introspection and penitence?
The process of purifying deeds and refining character traits naturally entails a certain dampening of the spirit. As we struggle to overcome negative personality traits, it is natural to lose some of life’s innate spontaneity and joy. The corrective process of teshuvah [repentance] can have the undesirable side effect of impairing the soul’s positive and creative forces.
This phenomenon is analogous to a patient who underwent arduous chemotherapy treatment in order to eliminate a cancerous growth. The therapy in fact eradicated the deadly growth, but it also weakened healthy powers of the body.
Therefore, the holiday of Succoth — a time of elevated spirits and holy rejoicing — immediately follows the introspective Days of Awe. The Succoth festival restores the soul’s wholesome sense of joy in life, and, in fact, completes the process of repentance and atonement.
(Silver from the Land of Israel, pp. 87-88. Adapted from Olat Re’iyah vol. II, p. 368)