True Happiness

Abaye was the head of the yeshiva, and when a student finished a masechta he made it a Yom Tov for the Chachamim. Rashi goes out of his way to tell us who Abaye was. Why? A similar puzzle sits in the Navi, where David dances before the Aron and his wife Michal calls him an empty-headed man. What was she afraid he was losing, and how could David answer, "It is before Hashem"? The Kedushas Tzion's answer also takes in two questions about Sukkos: why it is the happiest Yom Tov, and why we read Koheles.