Rabbenu Bachaye answers the question. He compares the seven days of Sukkos to the seven days of Pesach. The 8th day - Shemini Atzeres - is compared to Shavuos. The Ramban describes the counted 50 day period between Pesach and Shavuos as the ‘Chol HaMoed’ between them. It is for this reason that Chazal call Shevuos - Chag Ha’Atzeres. The same name used for the 8th day of Sukkos. This - says Rabbenu Bachaye - teaches us that the 8th day of the Chag (holiday) is equivalent to Matan Torah – the day we received the Torah.
Added to that is what the Eglei Tal says. There are some who say that if one learns Torah deeply because he enjoys it and rejoices in it… that is not considered learning Torah L’Shma – for its own sake. This attitude is a mistake. On the contrary says the Eglei Tal. The primary Mitzvah of learning Torah L’Shma is to rejoice in it. Because only then will it be absorbed it into one’s blood and be attached to it.
Therefore success in Torah learning is directly related to one’s Simcha – one’s joy. There is no greater time to appreciate that than Z’man Simchasenu the descriptive name given to the entire Yom Tov of Sukkos all the way through Shemini Atzeres. We rejoice - with the Torah.
Since we are already in a state of joy from the Chag which is maintained as we complete the yearly Torah reading cycle and return immediately to the beginning of that cycle - it helps to assure that we will be successful in our own Torah studies during the coming year.
Taken from Torah L’Daas
Taken from Torah L’Daas