Tanya Chapter 39 part 3

Normally, a soul who served Hashem with instinctive love and fear abides in the world of Yetzirah, as does the “ruach” of a righteous person’s soul. However, on special occasions the soul gets to rise to a world beyond its normal self, a privilege not available to angels. Why this special treatment? Because an angel doesn’t have to fight a dark side in order to implement its natural emotions. But a soul clothed in a physical body must resist evil in order to access its natural holiness, and that is tremendous!