Supernatural Thoughts and Quotes or How big of an asshole God really is

4x03 In the Beginning

Castiel to Dean: "Don't be too hard on yourself, you couldn't've stopped it. [...] Destiny can't be changed Dean. All roads lead to the same destination. [Dean: then why did you send me back?] For the truth."

Cas sends Dean back in time(to 1973) so that he can see with his own eyes what exactly happened to hisparents and grandparents and why Mary made the deal with Azazel. Throughoutthis entire time Dean still believes, still hopes he can stop what happened tohis family. He is proven otherwise. Castiel (on behalf of God) tells Dean thathis fate can't be changed, can't be altered. It's irreversible even if time isjust a construct. 

This, all of this, like everything else on this show inrelation to God can be attributed to the concept of free will. In God's worldfree will is an illusion. Sure you have some choices that are only chosen byyou and you have control over the decision making, but the motives andcircumstances of this decision are not under your control and even if, like Cassaid, everything will lead up to the fate God has chosen for you. So for Deanand Sam this means that they have decision-making power over a few things intheir lives, but only to the extent where it still leads up to the story andthe plot God is trying to tell through their lives. God shows Dean again thathe simply has to accept that the majority of his life isn't actually his makingand the power over his future and where he stands lies in the hands of others(God). 

This results in the realization of Dean to not fight what lies in hispath but to accept and resign himself with it. It just makes it easier for Godto use Sam and Dean as his marionettes who won't fight back or question thesituations they're confronted with. They just need to play their parts as bravelittle soldiers without a mind and a free will of their own as it'sdemonstrated to Dean in this episode.