The Shem MiShmuel (5673) teaches that, although everyone sits in the succah, only those who are worthy can experience its great joy and holiness. The Shem MiShmuel explains, "If the succah accepts a person, he will experience a good feeling while in the succah. But chas veshalom, if the succah doesn't accept him, he will not be happy there. If one enjoys sitting in the succah, it is a sign that the succah accepted him. This can't be seen. It is the feelings of the heart and revealed only to Hashem." The Shem MiShmuel compares it to teivas Noach, which didn't accept the animals that sinned in it.
In the yotzros of the second day of Succos, we say, מצותיה קיימו ִּיַמע כי ועד היודע ואנכי, "I [Hashem] know and testify that My nation kept the mitzvah [of sitting in succah]." The Shem MiShmuel asks, why does Hashem need to testify? Isn't it something that everyone sees?
The Shem MiShmuel answers that it is visible that we kept the external aspect of the mitzvos. Hashem testifies that we kept the inner spirit as well, that as we sat in the succah, we experienced the joy and holiness of the succah.
The Shem MiShmuel adds that Succos comes right after Yom Kippur because one needs to be tahor from all sins to be spiritually accepted into the succah.
Succah for Everyone
Nevertheless, let us encourage all those who don't yet enjoy being in the succah, by letting them know that they too accomplish a great deal just by being there. Rebbe Avraham Yaakov of Sadigura zt'l said that this is why a succah is made from גורן פסולת ויקב, the leftover branches of the vineyard and the wasted stalks of crops (see Succah 12.). It teaches us that the succah is even for those who feel they are פסולת, leftover, and unimportant.
Chazal (Succah 22.) say כשרה בית כמין המעובה, if the schach on the succah is very thick and appears like a house, it is also kosher. Tzaddikim said that this alludes to the Yidden, who are בית כמין מעובה very thick, coarse, and gashmiyosdig. The succah is for them, too.
There was a chassid who was afraid to walk into the succah. He said, "The succah is filled with Hashem's names. How can I step on them?" His Rebbe told him, "A Yid is also comprised of Hashem's names. Sheimos may go on top of sheimos."
Rebbe Yechiel of Alexander zt'l said, "It is truly a wonder how one can enter a succah. But every Yid is a ממעל אלוקי חלק, a part of Hashem. Hashem's name is within him. Therefore, he may enter the succah."
The Torah says (Vayikra 23:42), תשבו בסוכות, "Dwell in the succos..." תשבו has the same letters as בושת, shame. The Yeitev Lev (Yeitev Panim Succos 10) explains that one should feel embarrassed to enter the holy succah. How does he have the audacity to enter such a sacred place? The answer is with thoughts of teshuvah. As the pasuk continues, האזרח כל. The word האזרח can mean "I will shine." It is in the future tense – "Although I am not worthy to enter the succah right now, in the merit that I plan to improve my ways, I am able to enter the succah."
The pasuk concludes, בני את הושבתי בסוכות כי מצרים מארץ אותם בהוציאי ישראל, "I had Bnei Yisrael sit in succos when I took them out of the land of Mitzrayim." When they left Mitzrayim, they weren't worthy either, but in the merit that they would receive the Torah in the future, they were able to sit in the succah. Similarly, because I plan to improve my ways, I have the right to be in the succah, even now.