Gimel To clarify this concept: the Possuk says: עִּבְּדוּ אֶת־הַ׳ בְּשִּמְּחָּׂה “Serve Hashem with joy”, which is an instruction to all Yidden that their Avodah shall be with Simcha. And Simcha is a fundamental principle in Avodah.
As the Rambam writes at the end of the laws of Lulav: The Simcha, that a person shall ‘rejoice’ in the performance of a Mitzvah and [also] in the love of Hashem who commanded that Mitzvah is a great Avodah. [Furthermore] anybody who desists from this Simcha is worthy of punishment. As the Possuk states: תַחַת אֲשֶׁר לֹא־עָּׂבַדְּתָּׂ אֶת־ה׳ אֱלֹקֶיךָ בְּשִּמְּחָּׂה וּבְּטוּב לֵבָּׂב “Because you did not serve the Hashem, your G-d, with happiness and with gladness of heart”.
This means that even when a person has his Avodah in order, and he only lacks the Simcha he deserves a very serious punishment as if he did not even do the Avodah at all [!] This is because Simcha is fundamental to Avodah.
However, according to this we now have to clarify what the Possuk says: שִּמְּחוּ צַדִּיקִּים בַה׳ “Rejoice You Righteous Ones with Hashem” implying that it is only the Righteous who get to serve Hashem with Simcha.
