The Gemara relates: Mar, son of Ravina, made a wedding feast for his son and he saw the Sages, who were excessively joyous. He brought a valuable cup worth four hundred zuz and broke it before them and they became sad.
The Gemara also relates: Rav Ashi made a wedding feast for his son and he saw the Sages, who were excessively joyous. He brought a cup of extremely valuable white glass and broke it before them, and they became sad.
Similarly, the Gemara relates: The Sages said to Rabbi Hamnuna at the wedding feast of Ravina: Let the Master sing for us. He said to them, singing: Woe unto us, for we shall die, woe unto us, for we shall die....They said to him: What shall we respond after you? What is the chorus of the song? He said to them, you should respond: Where is Torah and where is mitzva that protect us?
In a similar vein, Rabbi Yoḥanan said in the name of Rabbi Shimon ben Yoḥai: One is forbidden to fill his mouth with mirth in this world, as long as we are in exile (ge’onim), as it is stated: “When the Lord returns the captivity of Zion we will be as dreamers” (Psalms 126:1). Only “then will our mouths fill with laughter and our lips with song” (Psalms 126:2). When will that joyous era arrive? When “they will say among nations, the Lord has done great things with these” (Psalms 126:2). They said about Reish Lakish that throughout his life he did not fill his mouth with laughter in this world once he heard this statement from his teacher, Rabbi Yoḥanan.
3. Rebbe Nachman of Breslov (19th cent. Hassidic thinker)
It is a great mitzvah (obligation) to be happy, constantly.