Succos
"In succos you shall dwell for seven days... So that your future generations will know that in succos I made the Jewish people dwell when I took them out of the Land of Egypt." (Vayikra 23:43)
The Gemara (Succah 11b) says that there was a dispute about the meaning of this verse. Rabbi Eliezer says that these succos in which the Jewish people dwelt in the desert were the clouds of glory that surrounded their camp from all sides and guided and protected them on their way. Rabbi Akiva says that the Jews actually dwelt in small huts in the desert, as we do today.
On the surface, it seems that in Rabbi Akiva's opinion our succah has nothing to do with the clouds of glory; it is simply a reminder of the huts we dwelt in then. But then why was a mitzvah given to memorialize this trivial event?
The answer is that Rabbi Akiva also agrees that our succah reminds us of the clouds of glory. It reminds us of the huts the Jewish people lived in in the desert - but how were they able to live in such a dangerous place, full of wild animals, snakes and scorpions, with so little protection? Only due to the clouds of glory.
Rabbi Eliezer's opinion, that the succah itself represents the clouds of glory, presents a difficulty: The succah roof is made of the most worthless substances: leaves, grass and branches. It cannot be anything man-made that was useful for any purpose. How can the most worthless substance be a reminder of the clouds of glory, where were miraculous and priceless?
The answer is that the Gemara (Berachos 33b) tells the story of someone who embellished the traditional prayers, adding a long list of praises to G-d over the standard "great, mighty and awesome." Rabbi Chanina rebuked him, saying, "Have you finished all the praises of your master? This is comparable to a king who had a million gold pieces, and they praised him as having silver."
Here too, if we were to use some precious substance to symbolize the clouds of glory, it would be so far from accurate that it would be an insult to Hashem. But using the cheapest things is in fact the best way to represent the clouds of glory, because we always find that opposite extremes go together. The rich man gives to the poor man, while the middle-class man is not involved with either - because he does not need to take charity, nor does he have extra money to give to charity. The greatest man who ever lived, Moshe Rabbeinu, said, "We are nothing!" The humble man is the closest to Hashem, as the prophet says (Yishaya 57:15), "He is with the lowly and humble man." The Gemara cites a disagreement as to whether Hashem comes down to be with the humble, or raises up the humble to be with Him (Sotah 5a). Here too, the humble grass and leaves of the succah is the best reminder of the majestic clouds of glory.
One might ask: if the succah was meant to teach our future generations about the clouds of glory, why isn't there a specific mitzvah to tell this story to our children, as there is on Pesach? In order to fulfill this mitzvah on Pesach we make a special effort to arouse children's curiosity by distributing nuts and taking the seder plate off the table, so that they should ask the Four Questions. Why isn't this done on Succos?
The answer is that our going out of the house to eat and sleep in the succah is unusual enough to arouse children's interest. We can be sure that the children will ask, "Why are we eating out here?" And the father will answer and tell them the story.
When we sit under the roof of the succah, we should feel as if the clouds of glory themselves were above our heads. On Succos, we are all guests at Hashem's table. That is why there is a mitzvah to rejoice on Succos: because it is written, "Strength and joy is in His place." (Divrei Hayamim I 16:27).
The Four Species
"Let me know the way of life, being sated with happiness in Your presence, pleasure in Your right hand victorious." (Tehillim 16:11)
The Midrash (Vayikra 30:2) draws a connection between the end of this verse and the mitzvah of the Four Species: Do not read "sova" (sated) but rather "sheva" (seven). These are the seven mitzvos of Succos: the four species, the succah, chagigah offering and the joy offering. Why the necessity for both chagigah and joy offerings? Because we have emerged from the trial (of Rosh Hashanah and Yom Kippur) victorious. This may be compared to two parties who came before the judge. When they come out, how do we know which one was victorious? The one carrying the spear is victorious. Similarly, the Jewish people and the nations of the world each argue their cases before Hashem. When the Jews emerge carrying their lulavim and esrogim, we know that they were victorious.
Accordingly, we can explain the beginning of the verse in Tehillim: "Let me know the way of life, being sated with happiness..." The Torah says that we must rejoice with the lulav (in fact, the Rambam in Sefer Hamitzvos Shoresh 11 says that rejoicing with the lulav is part of the very description of the mitzvah), but Dovid Hamelech asked: how can I rejoice, if I am already rejoicing for the entire Succos? Hashem answers him: In Your right hand victorious - the happiness of the lulav is a different sort of happiness. It is a happiness of victory on your day of judgment, and therefore it can coexist with the happiness of the festival.