A person who desires to ascend up the spiritual degrees, to go from our reality that is felt in the five senses to the upper reality, has to change himself and his qualities. And for that he needs an external, additional force which he lacks.
He has just a tiny point of desire for spirituality and must receive everything else from outside. The entire sum total of these forces, which raises him from the current state to a more advanced state, is called “the donkey driver.” In Hebrew “donkey” is Hamor, originating from the word Homer, matter. The donkey is our matter, the will to enjoy, and it continuously goes through changes, causing us to ascend and advance along the spiritual path.
The force from Above influences our desire, changing it, and thus our “donkey” becomes constantly improved because the “donkey driver” is always working on him. When we reach the end of the path, our “donkey” really does become “white,” just like in the stories about the Messiah who arrives on a “white donkey.”
From the 2nd part of the Daily Kabbalah Lesson 1/11/11, The Zohar