Misha told EW that whatever transpired in Purgatory between Dean and Cas started off as an act of penance for Cas:
"[Cas] left Dean because he wanted to try to keep him safe. [Cas] being around is actually a liability in Purgatory, and he’s working to try to right the wrongs of his past doings.”
Episodes 7 and 8 will reveal a new threat that will play a huge part in Castiel's arc for the season:
“There’s something going on with Cas that neither Cas nor the brothers know. Cas is sort of being manipulated behind the scenes and he’s not aware of that and neither are the boys. We’ll see how that affects their relationship in the long run.”
Who do you think is manipulating Cas?