Don't Worry, Be Happy

Four times in the Torah the reference of “simcha,” happiness, is used in relation to the Pilgrimage Festivals of Sukkot (the Feast of Tabernacles), Shavuot (the Feast of Weeks) and Pesach (Passover).  None with regard to Pesach, one in connection with Shavuot (Devarim – Parshat Re’eih, Deuteronomy 16:11), and three for the holiday of Sukkot (ibid. 16:14, 16:15, Vayikrah – Parshat Emor, Leviticus 23:40).  Why is that?  Is Sukkot supposed to be happier than the other holidays?  Is Pesach not supposed to be happy at all?  One of the “happiness” verses in general says we should be happy on our festival, and it does mean all three of them, but what is so special about Sukkot that it warrants three out of four?