A popular teaching by Rabbi Nachman of Breslov, a 19th-century Chassidic Rabbi, is "Mitzvah Gedolah Le'hiyot Besimcha Tamid", "it is a great mitzvah (commandment) to always be in a state of happiness".
When a person is happy one is much more capable of serving God and going about one's daily activities than when depressed or upset.
[1] Jews often use simcha in its capacity as a Hebrew and Yiddish noun meaning festive occasion.
The term is used for any happy occasion, such as a wedding, bar mitzvah or engagement.
[2] It was also a slang term used in Jewish-American organized crime circles to refer to a pimp.