The Rema m'Pano teaches as follows: The size of a small succah is 7 x 7 tefachim, which is 49 square tefachim. The height of a minimum succah is ten tefachim high. This means there are 490 square tefachim in the succah. 490 is the gematria of תמים. In (Devarim 18:13( it says תָּמִים תִּהְיֶה עִם ה' אֱלֹקֶיךָ. The measurements of the succah hints to the important concept of temimus – which is a simple and pure belief in Hashem. The Imrei Noam )'ג' דסוכות אות ד( adds that the mitzvah of lulav also hints to temimus because it states וּלְקַחְתָּתֶּם לָכֶם בַּיּוֹם הָרִאשׁוֹן פְּרִי עֵץ הָדָר כַּפֹּת (23:40 Vayikra) תְּמָרִים וַעֲנַף עֵץ עָבֹת וְעַרְבֵי נָחַל and Chazal say that וּלְקַחְתֶּם is made of two words,לקיחה תמה )see Succah 34b and 37a), which can be translated as "take temimus". These should be the traits we seek to develop on this Yom Tov.
It states (Vayikra 23:42-43( ...בַּסֵּכֹּת תֵּשְׁבוּ שִׁבְעַת יָמִים לְמַעַן יֵדְעוּ דֹרֹתֵיכֶם כִּי בַסֵּכּוֹת הוֹשַׁבְתִּי אֶת בְּנֵי יִשְׂרָאֵל בְּהוֹצִיאִי אוֹתָם מֵאֶרֶץ מִצְרָיִם "For seven days you shall live in succos.... in order that your ensuing generations should know that I placed Bnei Yisrael in succos when I took them out of the land of Mitzrayim."
The Gemara (Succah 11b) states a machlokes: Reb Eliezer says that the succos we commemorate are the ענני כבוד clouds of glory, which surrounded and protected Bnei Yisrael in the desert. These are called the "succos" that Hashem placed Bnei Yisrael in when they were in the desert. Reb Akiva says סוכות ממש עשו להם Klal Yisrael made actual succahs, like the ones we build on our porches and backyards for Succos. They were the temporary houses the Jewish people lived in during the forty years they were in the desert, after leaving Mitzrayim.
According to Reb Akiva, we must understand the pasuk לְמַעַן יֵדְעוּ דֹרֹתֵיכֶם כִּי בַסֶכּוֹת הוֹשַׁבְתִּי אֶת בְּנֵי יִשְׂרָאֵל"in order that your ensuing generations should know that I placed Bnei Yisrael in succahs when I took them out of the land of Mitzrayim." Didn't they build the succahs and dwell in them on their own? Why does it state כִּי בַמֶּכּוֹת הוֹשַׁבְתִּי אֶת בְּנֵי יִשְׂרָאֵל, that Hashem placed us and had us sit in succos?
The answer is that even when a person builds a hut and lives in it, he must know that Hashem did it. Everything is from Hashem. The idea to build a hut, and the strength and ability to do so, are all from Hashem. This is the temimus we attain from living in the succah for seven days. We discover that everything is from Hashem; even what we do on our own is from Hashem. לְמַעַן יֵדְעוּ דוֹתֵיכֶם this emunah we must know, and we must teach it to the future generations. Everything that occurs is from Hashem.
The Rashbam explains the reason we must know, in all generations, that Hashem placed us in succos. It is to learn to appreciate what we have now. He writes, "I placed Bnei Yisrael in succahs in the desert for forty years, a place where there was no civilization or portions of land. Thereby, you will praise Hashem for giving you a portion of land, houses filled with goodness. You won't say in your hearts that your strength and the might of your hand granted you your wealth." We understand from the Rashbam's holy words that sitting in the succah helps us recognize the goodness we have today, in comparison to what was in the past. In the past, we lived in a desert, without civilization and a regular home. Today, we live so differently. Furthermore, the succah reminds us that everything we have today is from Hashem. Let us not think we attained it with the strength of our hands.
Chizkuni (ibid.) writes, "The Yom Tov is in the time when we gather in the crops. Lest people's hearts will become proud due to their homes that are filled with all types of goodness, and they might say that they earned all this wealth by the strength of their hand. From sitting in the succah, they will praise Hashem Who gave them a land and houses filled with all good." Once again, the lesson of succah is to know that everything we have is from Hashem.
The Rambam (Morah Nevuchim vol.3, ch.43, quoted in the Ksav Sofer ד"ה ולקחתם( explains the reason for the mitzvah of the four minim: "After the harvest, and after gathering in the crops, every person will praise Hashem, and they won't say that they earned all of this due to their strong hand."