“Ivdu es Hashem b’simcha — serve Hashem with Happiness” (Tehilim 100:2).
[People make] a mistake in the simple meaning. Of course you have to serve with simcha. But what does it mean with simcha? You have to serve with the simcha, with the fact that you’re happy. That’s the way to serve Hashem. When you have everything, why [don’t] you serve Hashem with that happiness and say to Him “Thank You Hashem!” — World and Torah (#199)