The YomTov of Succos represents multiple ideas within the Torah and the Mepharshim. The Torah tells us that the Chag recalls how we lived in these makeshift houses as we left Mitzrayim. (Behar 23:43) The Gemara questions whether we are commemorating the actual “houses” in which we lived temporarily or whether we are really referring to the Ananei Hakovod which divinely protected us when we were in the Midbar. (Gemara Succah, 11b) Either way, The Gemara concludes we are recalling an event which took place in Nissan, around the same time as Pesach, immediately after Yitzias Mitzrayim, Indeed, Chazal say that Succos would be more appropriately celebrated in the spring but we shifted it to the fall so it would not appear that it was simply a comfortable time to construct an outdoor dwelling. Rather, we are doing so when it is unusual, so our purpose is clear.
It’s interesting how everyone agrees that Pesach and Succos are largely commemorating the same events - leaving Mitzrayim, yet the Torah’s instructions of how we are to mark and celebrate these two Moadim are significantly different. Pesach has a Seder, complete with Lechem Ani and Marror to remind us of the bitterness of Shibud Mitzrayim. In fact, a cornerstone of Pesach is the idea of “Zecher” to remember how difficult things were for us in Mitzrayim. In contrast, it is only in connection with Succos where we are instructed “L’maan Yeiduh Ledoroseichem” – so that future generations should know. (Behar, 23:43) Moreover, Pesach is commemorated as Zman Cheiruseinu – the time of our freedom, whereas Succos is marked as Zman Simchaseinu – the time of our celebration.
Simchas Torah adds another layer of mystery to our YomTov. First, we already have a Moed devoted to receiving the Torah – it is called Shavuos. Biblically, Shavuos marks the counting of the days to the Korban Omer but by rabbinical decree we have long co-joined the celebration of receiving the Torah to Shavuos since they are close to the same date. Why then would the Rabbonim have devoted another YomTov to celebrating the Torah and connect it to Succos and Shmini Atzeres?
The fact is that we would be Mshubud to Hashem on the basis of Yitzias Mitzrayim alone. For that matter, we would be Mshubud on the basis of Briyas HaOlam. But as we have seen so well over the generations, being Mshubud is not enough for everyone, particularly our youth who are prone to impulsiveness and a “what’s in it for me NOW” mentality. Pesach is critical as it establishes our historic debt to Hashem and unified us as a people. We MUST remember where we came and come from. Zecher. But perhaps our Chachomim knew that may not be enough. The Torah cannot simply be a set of rules of what we must do and can’t do. Its staying power is in the Simcha that living a Torah life brings. The achdus of its people, the common love of Eretz Yisroel, the families and communities which are created by living according to its dictates. That has appeal to everyone.
Shavuos and Kabbalas Hatorah was where our lives as Torah Jews began. But the Torah itself tells us that L’maan Yeiduh L’doroseichem requires Simcha. Succos and Simchas Torah are that part of the equation.
Yes, when we go into the Succah we are commemorating the shelter which Hashem provided us in the Midbar. But we are celebrating his love for us, not simply remembering it. We are celebrating with each other – with our families and friends, not simply living amongst each other. We leave our houses, our computers, our telephones and spend eight days enjoying the YomTov with those that we love. We take the Arba Minim, all symbols of our bountiful harvest. Indeed, we have much to be thankful for and the Torah prescribes a time to stop and recognize it. Finally, we are ready for Simchas Torah – not Shavuos where we mark receiving the Torah by staying up and learning all night but by rejoicing our lives as Torah Jews. By appreciating the time out to be with family, literally immersed in the Succah and the associated Simcha.
Yitzias mitzrayim and Kabbalas Hatorah were great world altering moments. Hashem and our Chachomim foresaw that it had to mean much more for future generations to have a small semblance of the gift which we received. That gift was a guide for living fulfilling and happy lives in the service of Hashem. Not just a guide for serving Hashem but a guide for us to find true joy in that pursuit.
 The giving of the Torah actually took place on either Sivan or Sivan.
 The YomTov of Simchas Torah was a relatively recent enactment when we changed the cycle of concluding the entire Torah every three years to the now current annual basis.