Why exactly do negatives entities inevitably need to switch polarities?

[Loki]

Spiritual evolution results in losing boundary of individuality. Ra says he cannot distinguish individuals in Ra memory complex.
Negative polarity requires individuality in order to be possible. Once the spiritual evolution pushes negative entities above the individuality threshold, negative potential becomes impossible.
If someone wants to keep his/hers negative potential must stop spiritual evolution. But spiritual evolution is fundamental goal of each entity.