Be happy on Succos because Hashem is happy on Succos. The Sfas Emes (ימי ה"ד ג"תרמ) zt'l writes, "When the Jewish nation becomes pure from all their aveiros on Yom Kippur, there is immense joy in heaven, and we should participate in Hashem's joy."
The Alshich (Emor) writes, "From the day the world was created, Hashem was never as happy with His creation as the day the Mishkan was built. And in every generation, Hashem is happiest on Succos, for then Bnei Yisrael are cleansed from their sins. The succah is our sign that we attained this purity."
The Shlah HaKadosh (ט אות סוף מצוה נר סוכה) writes, "A person must be happy every day of Succos, his face should beam with joy because the holy Ushpizin dwell with him."
Also, we are happy on Succos because of all the mitzvos we perform on this Yom Tov. In Hilchos Lulav (8:15), Rambam writes, "The joy that one has when he performs mitzvos is a great service, and whoever refrains from that joy deserves to be punished."
