Our Universe's natural state is one of balance.  You might also hear the word "karma" thrown around here and there.
Karma is another word for self-imposed balance. In order to maintain balance as our souls progress back to Source, we may choose lives with opposing themes in order to balance out the growth experience.
For instance, one could choose an incarnation in which he distorted the concept of "oneness" by playing the part of the perpetrator, narcissist, or physically oppressive individual. Then, in another life, he could choose to explore a theme in which he experiences being the "victim," or the recipient of actions from negatively focused individuals.
Living lives on both sides of the dualistic spectrum allows a soul to experience the feelings, emotions, pain and joy associated with viewing life from very opposing paradigms.