Powtórzonego Prawa 28:47 Kabała: Sha'arei Kedusha

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.