[quote=@Memory]
<Snipped quote by Yatagarasu>

Is it the most basic stuff the soul will naturally move toward? It's familiar?
[/quote]

In part, yes. The goal of this method is to create a core that is compatible with the soul, and the reason we use an element compatible with the body is familiarity. But the reason we need a "start" is for the core's formation; we have to conceptually "progress" or else it will be stagnant and unable to feed the soul.