The Sefer Hachinuch suggests another reason behind the Mitzvah of Lulav and Esrog. The holiday of Sukkos is described in the Torah as Chag Ha’asif, the festival of gathering, as the holiday coincides with the harvesting of one’s produce. The harvested produce would naturally bring great joy to the owners. The taking of the Lulav and Esrog is a way of channeling the self-joy we experience at this time and direct it to G-d. Additionally, celebrations can at times take a turn away from the spiritual. By utilizing the same items that we are ecstatic over, we ensure that our celebration will maintain a spiritual flavor.