The pasuk states: "B'Sukkos teishbu. (You shall dwell in sukkos).”
The Yitav Lev zy"a (in Sefer Yitav Panim) notes that the word "teishbu" has the same letters as the word "boshes" (embarrassment). He explains that if a person would think about how holy the Sukkah is and how Hashem provides us with a "light that surrounds us" and raises us to great heights, he would be overcome with embarrassment for any sins he ever committed. He would be ashamed to enter this holy place knowing that he did bad things in the past.
The best solution for this problem is to accept to improve one's ways and not to do any more aveiros. When one does so, Hashem accepts him and considers it as if he already perfected himself. He is now worthy to enter the Sukkah.
The Yitav Lev continues by saying that the deeds of tzadikim are called "ohr" (light), while the deeds of the wicked are called "choshech" (darkness). With this in mind, he explains the pasuk that says that every “ezrach” (literally "every citizen") shall sit in the Sukkah. The word "ezrach" can be understood to mean “ohr zorach" - my light will shine. In other words, if someone promises to improve and who says that in the future he will be a tzadik whose deeds are "a light", he can then sit in the Sukkah without shame. The proof of this is "because the children of Yisroel sat in Sukkos when they left Egypt." Even though they were on the 49th level of impurity, Hashem took them out of Mitzrayim because they would accept the Torah in the future. So too, we can sit in the Sukkah without embarrassment if we accept to shine in the future by becoming tzadikim.
The Preciousness of the Sukkah:
Whenever Sukkos started on a Sunday night, the Bnei Yissochor zy"a would offer non-Jewish workers a lot of money to build his sukkah on that day. He would have to give them two pints of whiskey to convince them to work on their holiday (Sunday).
When he entered the sukkah at the beginning of Yomtov, he would spread out his hands and feet and kiss the floors and the walls. He would enthusiastically say, "Ribono Shel Olam, you see that for a bit of whiskey, non-Jews are willing to work on their holiday. But Your nation wouldn't do this for all the money in the world, even in this lowly generation."
With the Entire Body:
The Vilna Gaon zy"a would say that there are only two mitzvos that encompass the entire body – sukkah and living in Eretz Yisroel. This is hinted to in the pasuk (Tehillim 76:3): "Vayehi b'shalom suko u'maonaso b'tzion", which he translated to mean that the mitzvos that are done with "shelimus" (the entire body) are sukkah and living in Tzion.
Full of Holy Names:
The Sanzer Rov zy"a (quoted in Sefer Chakal Yitzchok, Parshas Eikev) also notes that the mitzvah of sukkah is the only mitzvah that encircles the entire body. It is true that tzitzis also cover the majority of the body and wrap one who wears them in holiness, but they do not cover the feet. When one is in a sukkah, however, even his feet are encircled by the mitzvah.
Furthermore, he says that the sukkah consists of "holy names" of Hashem, and even the ground of the sukkah is made up His holy names. Hashem commands us to sit in the sukkah and walk upon the holiness in order to purify and sanctify ourselves. Hashem considers this as if we went into exile...
A Bigger Mitzvah if Done by Oneself:
Sefer Shivchei Rav Yaivi relates that Rav Yaivi of Ostroha would do all the work related to mitzvos himself. Even when he had others who could do it for him, he would do it on his own. He would personally purchase schach, and make several trips to carry it home on his shoulders.
Sitting in the Sukkah is Like Leaving Golus:
The Yalkut (Emor, Remez 653) states: "Rav Eliezer bar Maros said: Why do we make a sukkah after Yom Kippur? Because on Rosh Hashanah, Hashem judges the entire world. On Yom Kippur, He seals the judgment. It is possible that it was decreed that we must be exiled. Therefore, we make a sukkah and exile ourselves from our homes, and Hashem considers this as if we went into exile..."
With Him in Gan Eden:
Sefer Siach Sarfei Kodesh relates that Rav Fishel of Strikov zy"a lived in the same house as a tailor. The tailor built a sukkah that he decorated with all types of paintings, flowers, fruits, etc. When the Strikover saw the sukkah, he asked who decorated it so nicely. When he was told that it was the tailor, he was very happy. He asked him if he wanted a bracha for children, wealth, or a long life. The tailor answered, “I only want one small thing. Just like we dwell together in this world, I want to be with you in the next world."
The Rebbe replied, "That's not a small thing. That's a major thing. But since I promised to fulfill your request, I can't retract my offer, even though it will be hard for me."
After the Rebbe's petirah, the tailor got sick. He passed away just eight days later.
The Best Sukkah Decorations:
Sefer Keser Shem Tov (Hosafos at the end of the sefer, Os 112) quotes the Baal Shem Tov zy"a as saying: "My sukkah decorations are the simple Jews who accept to serve Hashem with simplicity. Through the joy they have in the mitzvah, which comes from their simple emunah, they create much joy in Shomayim."
The same sefer adds (ibid, Ois 113) that a major part of the Besht's avodah on Sukkos was to bring joy to simple Jews.
Similarly, it is related that Rav Moshe of Razvodov zy"a had a beautiful sukkah, which was decorated very nicely. It was so nice that people would come from all over to see it. Even the government ministers of Vienna once came to have a look. Still and all, the Rebbe would humbly say that he envied the simple Jews who made sukkos of just four walls, solely for the purpose of fulfilling the mitzvah.
A Choleh is Not Exempt From Welcoming Guests:
A guest once came to the home of Rav Chaim Ozer Grodzensky zt'l in Vilna on Sukkos, and the Rov told him, "Please go to my sukkah in the backyard and eat as much as you want. Unfortunately, I am sick and am exempt from sitting in a sukkah, so I have to remain in the house."
The guest went to the sukkah. He was surprised when Rav Chaim Ozer appeared in the doorway and proceeded to sit down next to him at the table. The man asked, "Didn't the Rov say that he is sick and exempt from sitting in a sukkah?"
Rav Chaim Ozer replied, "According to halacha, I am not obligated to sit in a sukkah, but I am not exempt from the mitzvah of hachanasas orchim, as we see that Avrohom Avinu fulfilled this mitzvah even when he was sick. Therefore, I decided to sit with you in the sukkah, as it would be wrong for me to allow my guest to sit alone.”
To Merit Sitting in the Sukkas Livyasan:
Rav Levi Yitzchok of Berditchov zy"a would invite many simple Jews to his sukkah. He maintained an open door, with everyone being warmly welcomed. He explained this custom by saying, "In the future, when tzadikim will sit in the sukkah made from the skin of the livyasan, I will want to join them. If they ask me why I should be permitted to sit with the tzadikim, I will have an answer. I will say that I allowed simple people into my sukkah, so even though I am a simple man, I should be allowed into this sukkah."