Quote Originally Posted by Squall of SeeD
However, Sephiroth's will is -- as was said by Tetsuya Nomura in The Distance: The Making of Final Fantasy VII: Advent Children -- above all and second to none. He refused to allow himself to be absorbed into the Lifestream to simply become a memory. As such, his consciousness -- as it dispersed through the Lifestream's Spirit Energy -- contaminated the surrounding Spirit Energy with his will, and he then made himself manifest once more, though he was now divided into three seperate entities, all with seperate aspects of his consciousness, and, thereby, their own unique personalities.
There you have it, straight from Nomura himself.