Why exactly do negatives entities inevitably need to switch polarities?

[Aion]

Found this tasty quote on this topic, which in essence, I think gets to the heart of the matter.

85.11 Questioner: Then the service-to-others path have potentiated that which is not. Could you expand that a little bit so I can understand it better?

Ra: I am Ra. If you see the energy centers in their various colors completing the spectrum you may see that the service-to-others choice is one which denies the very center of the spectrum—that being universal love.

Therefore, all that is built upon the penetration of the light of harvestable quality by such entities is based upon an omission. This omission shall manifest in fourth density as the love of self; that is, the fullest expression of the orange and yellow energy centers which then are used to potentiate communication and adepthood.

When fifth-density refinement has been achieved, that which is not is carried further, the wisdom density being explored by entities which have no compassion, no universal love. They experience that which they wish by free choice, being of the earnest opinion that green-ray energy is folly.

That which is not may be seen as a self-imposed darkness in which harmony is turned into an eternal disharmony. However, that which is not cannot endure throughout the octave of third density, and, as darkness eventually calls the light, so does that which is not eventually call that which is.