my theory is simply this: Castiel is being groomed to be the next God.
a few facts supporting my theory: (keep in mind i've only watched up to season
-Castiel has been raised by god at least twice (god did not however raise Sam, suggesting he isn't as important)
-Castiel is the obvious choice to succeed god as he cares more about humanity than any other angel we've met
-death said eventually he would reap god
-on more than one occasion it is made clear (particularly through Joshua) that god no longer cares about humanity; that he's tired
would it not make sense that god is scouting out a new leader for his creations before he checks out? Castiel has been through many trials, he learned a lot about free will and has made many, many mistakes along the way, but has learned from them. In my opinion this is because he is being groomed.