Kabbalah sa Mga Awit 100:2: Zohar & Sha'arei Kedusha

R. Isaac opened his discourse with the text: “Serve the Lord with fear and rejoice with trembling.” We also read: “Serve the Lord with gladness, come before him with singing.” These verses seem to contradict one another. But the truth is that “Serve the Lord with fear” means that all service which a man renders to his Master should begin with fear and awe of Him, and through this fear he will come to perform the precepts of the Torah with joy.

The opposite is true when one serves HaShem-יהו״ה, blessed is He, with joy, as it states,⁴¹⁸Psalms 100:2 “Serve HaShem-יהו״ה with joy.” For joy increases one’s desire and love of adhering to HaShem-יהו״ה, blessed is He, as written,⁴¹⁹Deuteronomy 28:47 “Because you did not serve HaShem-יהו״ה your God with joy and gladness of heart etc.” For, a person who serves HaShem-יהו״ה, blessed is He, with melancholy, is likened to a servant who serves his master with a sullen and annoyed face.