The Book of Zohar, “VaYikahel” Item 131: This is so because after the separation ZA was established as entirely Hassadim, on the right line, and Malchut as all Gevurot, on the left line. Hence, they are disputed as in the two lines, the right and the left of Bina, until ZA ascends in the middle line and establishes peace between them, uniting them with each other.
This is what our ascent depends on, on the extent that we can attribute everything, all the reasons, to one cause, to one state, to the middle line. The middle line is not found in nature; it comes from the Light that Reforms.
If we try to establish the middle line by ourselves, then the left line is revealed in contrast to it, as it says: “He who is greater than his friend (on the right line), his desire is even greater (than the left line).” Then the two lines converge into the middle line, so we ascend.
From the 4th part of the Daily Kabbalah Lesson 12/25/13, Writings of Baal HaSulam