Q. Why is Sukkot called “Our Festival of Joy”? Surely all the festivals are joyous?
A. The name derives from two Biblical references to joy on Sukkot, v’samachta b’chaggecha (Deut. 16:14) and v’hayita ach same’ach (Deut. 16:15). Because of these words, the sages say it is a special duty to rejoice on Sukkot.
In addition to the sukkah, there is the additional mitzvah of lulav and etrog, in relation to which the Torah says, us’machtem, “and you shall be joyful” (Lev. 23:40).
Yes, we also greet each other on Pesach and Shavu’ot, not only Sukkot, with Chag Same’ach, but the rejoicing on Sukkot is greater because the harvests of the year are successfully completed whereas on Pesach and Shavu’ot it is uncertain whether there will be a good crop.
Other answers to your question include the extra joy of the festival of the water-drawing on Sukkot and the belief that the sukkah symbolises the happiness of being covered by the ananei kavod, the clouds of glory, with which God protected our ancestors in the wilderness.