The secret to true happiness

Parshas Ki Savo ends its long list of curses with a surprising reason: because you did not serve Hashem with joy. But where in the Torah are we commanded to be joyful, and how can we be punished for missing something never asked of us? The Rambam in Sefer HaMitzvos, in the third mitzvah, offers a place to look. Before Rosh Hashanah, a Torah scholar comes home to find his house turned upside down and a book fund gone. His reaction startles the neighbors, and the story keeps going. What would it take to be happy no matter what?