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

You were on the right track, just thinking too physically. It's because it's the "first." Yes, it can turn into other elements physically through various processes, but souls don't work off of physical matter. So we need something to represent "one" or "beginning," for the process, and the first element formed in nature works well for that.

Knowing that, I bet you can guess some of the other materials' purposes.
[/quote]

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