In Torah study, it is apparent that mystical union is achieved, not by emotional identification but by intellectual identification with the text.
It is a matter of union of thought with thought, human cognition of God's "thinking."
And as a result there is a nullification of the soul before the Divine.
But beyond this intellectual integration there is another, more profoundly penetrating riddle.
If God is all and possesses all the greatness and the glory, what can one give back to God in gratitude?
As it is expressed in the Bible: "From Yours have we given unto Thee" (I Chronicles 29:14).
If one is indeed decent and upright, then one returns to God something of the fruit of the earth that was provided by Divine bounty.
What else can one give? What is so worthy in God's eyes that He would receive it with joy?
The answer is one's very self.
Not something that be, longs to me, but my very self. As long as a person considers himself to be "somebody," he has a self that is independent of God, to a greater or lesser degree, and he owns things that are his and his alone.
And, in the ordinary course of life, he may part with these as he sees fit.
But the giving away of that which is one's soul, the self of the self, is an act that exceeds anything that can be done by even the highest celestial beings.
The heavenly hosts simply do not have it to give away.
Man, however, has a soul, a sense of independent selfhood, which he can offer up to God as a sacrifice. This is the most valuable of all the possible gifts conceivable, in Heaven or on earth.
–Rabbi Adin Steinsaltz
From "The Way of the Soul and Torah" in The Candle of God by Rabbi Adin Steinsaltz