Unity
Sfas Emes (Yom Kippur 7651) writes, "On Yom Kippur, Bnei Yisrael become united. This is because we are all united in the realm of the soul. It is sin that causes separation. On Yom Kippur, when our aveiros are forgiven, we become one."
The mitzvah of succah is a result of the unity we have attained. As it states, האזרח כל בסוכות ישבו בישראל and the Gemara (Succah 27) explains, "all Yidden can dwell in the same succah." When there is machlokes, people don't want to be under the same roof or sit at the same table. But on Succos, the Torah tells us, "All Yidden can dwell in the same succah." This represents the achdus, unity, attained through the forgiveness of aveiros on Yom Kippur.
The four minim also represent unity among Yidden. The Midrash (Vayikra Rabbah 30:12) teaches that the esrog, lulav, hadasim, and aravos embody all levels of Yidden, from the tzaddikim (represented by the esrog) to those who are at low spiritual levels (represented by the aravah) and all Yidden in between. They are tied together to perform a mitzvah because they need one another. The aravos (the lower Yidden) must be close to the esrog (to the tzaddikim) because their connection with the tzaddikim grants them atonement. Tzaddikim need to be connected to the lower people because they aren't complete on their own. The Beer Heitev (645:9) writes, "אתרוג is gematriya 610. When combined with the other three species, the gematriya is 613." Perfection is achieved when all four categories of Yidden unite.
