There is a love of God that surpasses all the forms of love the Tanya has described so far. Rabbi Schneur Zalman of Liadi compares it to gold surpassing silver. It burns like fiery coals.
The previous forms of love emerge from the right side of the divine, from chesed (חסד), lovingkindness. They are characterized by longing, by yearning to draw close. The Hebrew word kesef (כסף), silver, shares a root with the word for longing, as in "you longed for your father's house" (Genesis 31:30). But this higher love comes from the left side, from gevurah (גבורה), the divine attribute of strength, rooted in binah (Understanding) ila'ah (בינה עילאה), supernal understanding.
When a person contemplates the greatness of the Ein Sof (אין סוף), before whom everything is truly as nothing, the soul catches fire. It does not merely yearn to draw close. It surges upward, striving to leave the body entirely, like a flame leaping off a wick, desperate to be parted from the wood that holds it down. First comes thirst: "My soul thirsts for You" (Psalms 63:2). Then lovesickness (Song of Songs 2:5). Then the rapture of complete soul-expiry: "My soul pines away" (Psalms 84:3).
This is the spiritual root of the Levites, whose Temple service was to raise their voices in melody and thanksgiving. Their song embodied ratzo va-shov (רצוא ושוב), the ecstatic "advance and retreat" of the soul, rushing toward God and pulling back to remain in the body, like the angels described by Ezekiel, whose movement is "like the appearance of a flash of lightning" (Ezekiel 1:14).
The Tanya admits it cannot fully explain this love in writing. But it promises that every person of warm heart and deep intelligence who binds their contemplation to God will discover it hidden within their own soul, each according to their capacity.