Unity
One of the segulos of succah and the four minim is that they are mesugal for אחדות, that there be unity among Yidden, and that there shouldn't be any machlokes.
The Gemara (Succah 27) says, "All Yidden can dwell in the same succah." There are halachic lessons that are learned from this phrase.26 Nevertheless, there is also a hinted message. It tells us that Klal Yisrael will have the attribute of unity. Where there isn't unity, or chalilah, if there is machlokes, people aren't able to sit under the same roof or to eat at the same table. Yet, the Torah tells us, "All Yidden can dwell in the same succah." This means there is unity among them, and the unity comes from the mitzvah of succah. It is mesugal for annulling all machlokes and for attaining unity among Yidden.
This unity began on Yom Kippur. Sfas Emes (Yom Kippur 7651) writes, "On Yom Kippur, Bnei Yisrael become united. We are always united in the realm of the soul, and it is sin that divides us. On Yom Kippur, when our aveiros are forgiven, we become one." This oneness is experienced on Succos.
The Midrash (Vayikra Rabbah 30:12) teaches that the esrog, lulav, hadasim, and aravos represent all Bnei Yisrael, from the tzaddikim (represented by the esrog) to those who are at low spiritual levels (represented by the aravos) and all Yidden in between. They are tied together. This means the aravos (the lower Yidden) must be close to the esrog (to the tzaddikim). Their connection with the tzaddikim grants them atonement. The tzaddikim also gain from their connection with all Yidden, including those who are at levels far beneath them. As the Beer Heitev (645) writes "אתרוג is gematria 610. When combined with the other three species, the gematria is 613." Perfection is achieved when all four categories of Yidden unite.27
26 The Gemara explains that "All Yidden can dwell in the same succah" teaches us that a succah isn't required to be לכם, owned by the person performing the mitzvah. The mitzvah can also be performed with a borrowed succah.
27 A rasha said to Reb Meir of Premishlan zt'l, "The Rebbe is like an esrog, and I am like the aravos. Both of us are needed. If someone buys the most precious esrog but doesn't have aravos, he cannot perform the mitzvah. So, what makes you better than me? We both need each other, and we are both lacking when we are alone!"
Rebbe Meir of Premishlan replied, "Certainly, we should unite together. That is ideal, and that is what we do on Succos. However, when reshaim group together and distance themselves from the tzaddikim, this is like a bunch of aravos on Hoshana Rabah; we hit and bang them until their souls depart..."
The Chofetz Chaim zt'l notes that the esrog, which represents the tzaddikim, isn't tied together with the other species. They are held together for the mitzvah, and then they part ways. This hints that although tzaddikim should connect to others to elevate them, they should limit their interactions with people of lower spiritual levels, so that they won't learn from their ways.