Rabbi Israel Baal Shem Tov explained that physical coupling and procreation cannot take place without arousal, passion and joy. The same is true of spiritual “coupling”.
Torah study and prayer must be experienced in a state of arousal, passion and joy if they are to bear fruit…Spiritual devotion directed toward building intimacy with G-d will be an experience of sweetness, delight and joy….
It is written, “I will rejoice rejoicing in G-d” (Isaiah 61:10). Rabbi Dov Ber, the Mezricher Maggid explained: The prophet is teaching: “I have joy and delight from the fact that I am worthy of perceiving the pleasantness of G-d and delighting in His worship. And from this itself I have joy – in that I am worthy of coming close to G-d in this manner and delighting in His worship.” Therefore, the prophet writes, “I will rejoice rejoicing in G-d.” That is, I have joy and delight from the very fact that I am “rejoicing in G-d.”
Adapted from “Adventures in Hassidic Thought” by Rabbi Aryeh Kaplan