The Torah tells us: “Serve God with joy”.
For most of Jewish history we took that to mean “Be happy while serving God'”
About 200 years ago, the Baal Shem Tov taught us that “If you want to serve God, be happy!” Not that while serving God we should be happy, but rather, simply by being happy we serve God.
Not just in doing Mitzvot, but in life, happiness is not optional. We are obligated to be happy.
When we are happy, its the ultimate service to God.
Chassidim used to say, that although sadness isn’t a sin, but it will drag you down lower than any sin, and although happiness isn’t a Mitzvah, it will lift you up as much as any Mitzvah