Zman Simchaseinu

Why do we read Koheles, with its cry of "hevel havalim," on Sukkos, the holiday the Torah calls Zman Simchaseinu? And why would the Torah send us out of our comfortable homes into a shaky hut at the very moment we are meant to be happiest? Some say Koheles is read to keep the joy from going too far. The Gemara in Shabbos shows Koheles praising simcha in one chapter and dismissing it in another. Which kind of happiness is being praised, and which one dismissed? Everything we chase this season hangs on the answer.