The Baal Shem Tov comments on the verse "The Lord is your shadow upon your right side" (Psalms 121:5), "God acts, as it were, as our shadow."
When a person lifts his hand, the shadow of his hand also rises.
Similarly, when a person acts kindly, God correspondingly acts kindly.
When a person acts to the fullest capacity of his ability, God completes his intent, in its entire breadth and fullness.
The battle between good and evil within man rages to the extent that God permits it.
Evil that continues to exist in the presence of good is an anomaly–yet God allows that anomaly to exist because it is the true means of human elevation.
Just as light defines darkness, so does darkness define light:
"Good delineates evil, [and] evil delineates good." (Sefer Yetzirah)
Oneg, or "delight," is the highest level of holiness–yet its letters, when reversed, spell nega,"plague." (ibid.)
The polarities of good and evil, of black and white, define each other, making up a single and interdependent construct.
God allows this structure to exist as long as it is needed.
At the moment that it is no longer needed, it will cease to exist.
–Rabbi Adin Steinsaltz
From Understanding the Tanya, Chapter 29, by Rabbi Adin Steinsaltz