Musar su Salmi 68:4

JOY: The second [branch of love of G-d] is joy, it is a great, essential principal in serving G-d. This is what David exhorted us saying: "Serve G-d with joy, come before Him with song" (Tehilim 100:2), and "the righteous will rejoice, they will exult before God and delight with joy" (Tehilim 68:4). And our sages said: "the Divine presence rests on a person only through his rejoicing in a mitzva" (Shabbat 30b). On the aforementioned verse: "Serve G-d with joy", our sages said in a Midrash (Shocher Tov, Tehilim 100): "Rabbi Abahu says: 'when you stand to pray, your heart should rejoice, for you are praying to the Almighty of whom there is none like Him'".