The joy of Sukkot is the other side of the coin of the anguish of Yom Kippur. On Yom Kippur we focus on the anguish entailed in our never ending battle with the evil inclination, on the many defeats that we inevitably must suffer as we go through life endlessly doing battle with him. But on Sukkot we focus on the glory that accrues to us as a result of the difficulty involved in this very struggle. This glory is the source of the joy felt both by God and by Israel. Sukkot takes place at the time of the harvest.
Many people think of it as the Jewish harvest festival.
After the anxiety of the planting and the growing, suffering through all the uncertainties that could affect the successful outcome, when you finally have your produce successfully gathered and safely stored in your barn, you can finally turn your mind to enjoying the beauty of what the world has to offer. The time of the harvest is when it is the easiest to stray and desert one's Maker. The joy of success and one's full stomach leads one to admire the beauty of the sunset and the many wondrous aspects of being alive. This joy is what distracts man from his battle with the evil inclination and allows him to forget that he is only a temporary resident in this life, and the transitory nature of all his non-spiritual experiences. It looks for nature's most beautiful fruit, the citron, nature's most beautiful leaves and takes these and rejoices with them before God.