Kabbalah zu Hoschea 9:1

(35) And from the Tanna'im regarding the happiness on a festival [we learn] that one should not be happy with laughter and frivolity like the idolaters in their festivals, as it is written "Rejoice not, oh Israel, with joy as the peoples" (Hosea 9:1), rather, one should be happy simply because it is a festival, as they are days of holy gatherings, as we explained above. One should feel like a person who is standing in the dark, that feels happy when they see new light, so too one needs to be happy through Torah in the holy days more than in regular days, as our rabbis explained "holy days were only given to Israel so Israel would busy themselves with Torah" (Yerushalmi Shabbat Perek 15, Halacha 3).