Yalkut, Emor 654
You find three verses that command to rejoice on Sukkot . . . For Passover, however, you will not find even one command to rejoice. Why not? Because in the season of Passover, judgment is being passed on field crops, and nobody knows whether the year will bring forth crops or not. So, too, you find that for Shavu'ot there is only one command to rejoice . . . Why? Because the field crop has already been brought into the house. Then why not two commands to rejoice? Because judgment is then being passed on fruits of trees. For Sukkot, however, when both field crops and fruits of the tree have been brought into the house, three commands to rejoice are set down.
We are blessed with so much—until we are not. Which is why we don't count chickens before they hatch or rejoice before the crops are in: it ain't over till it's over.
Sukkot comes on the heels of Yom Kippur, and while there is a tradition that holds that the gates of judgment are not completely shut until Hoshanah Rabbah in the middle of the week, a more mainstream approach is that Ne'ilah ends the Season of Repentance. How fitting then that the holiday of celebration, designated by three separate commands to "rejoice," should come only after those gates have closed.
Moreover, it is not until it is time for Sukkot that the crops and the fruit have been "brought into the house." Having committed to our new year's resolutions on Yom Kippur, we have needed these few days to internalize—"bring home"—all that we've set out to do.
After spending the month of Elul in spiritual preparation for the Yamim Nora'im, and then observing Rosh Hashanah and the ensuing Ten Days of Repentance, capped off by Yom Kippur, we feel ready to begin again. The joy that comes with that new beginning is exuberant, meriting a holiday designated by a threefold command for celebration.