Incarnations occur naturally and is the way forming souls get their start. The stuff of souls is everywhere, disbursed throughout the Universe, but so disbursed it does not make a soul. The stuff of souls tends to gather in living things as the environment is more interesting than non-living environments. It is not so much attracted to life as that it lingers, and thus begins to accumulate.
Without conscious intelligence it fails to establish a personality, a self, and after death of the life form disburses again. Incarnations happen naturally at first, as having formed a self the forming soul migrates to a new body after the death of the old.
It can happen in unguided migrations that more than one forming soul wants to inhabit the same body, but usually cohabitation is dangerous and distressing to both parties, including the body, and results in only the strongest soul remaining.
Unsparked souls are chaotic, aware of themselves as separate entities from other humans or animals, but basically dazed and wandering about, open mouthed. They are taking it all in, as the first passage, the first tour, the first time visiting a place.
Unless some matter occurs to spark a soul, such as empathy for another in distress and a choice to sacrifice comfort of the self to assist the other, no sparking occurs. A lazy life, lived in a local indolent with simple tasks, or taken out by early childhood illness, will not spark.
There is a greater percentage of unsparked souls in those continents or places where life is a struggle, not industrialized, not in high communication with other cultures.
Third World countries have a higher percentage of unsparked souls. Rural areas have a higher percentage of unsparked souls than urban areas. Thus, the corridors of power almost invariably have older souls.