Spiritual Ingathering
The physical world reflects the spiritual. Sukkot, which is literally the season for gathering in the produce of the fields, is also a spiritual Festival of Ingathering. All the Torah, mitzvot and good deeds that a person acquires throughout the year are gathered together on Sukkot.
(Shem MiShmuel)
The Body and the Sukkah
When Rabbi Mordechai of Lechovitch would enter his sukkah on the first night, he would fall to the ground, kiss it, and lament, "how can a coarse, physical body dare tread upon this beloved mitzvah, the mitzvah of sukkah!"
Uniting the Small and the Great
When the hadassim and aravot are bound together with the lulav, the mitzvah is complete. This alludes to the idea that when the small joins together with the great, completeness is achieved.
(Chiddushei HaRim)
The Lulav
The lulav symbolizes the Jewish people. Although it has many leaves, they cling closely to one another, and it has only one heart, central spine. So too, although the Jewish people consist of many tribes and diverse communities, they are united "as one person with one heart" to fulfill the will of their Father in Heaven.
(Yeina Shel Torah)
Joy Without Calculations
Joy does not need to arise from various considerations or calculations. Rather, "you shall rejoice before Hashem your G-d"—simply because He is Hashem, our G-d. The very fact that Hashem wants us to be joyful should itself bring us to joy.
(Rabbi Avraham of Slonim)