“Purgatory is a really brutal, free-wheeling place, but it has different effects on different people. It will have a different effect on Cas and Dean as you already started to see [in the Season 7 finale]. But I will say what you think you saw might not be exactly what you saw.”
I just had a thought. What if Purgatory effects people by tapping into their fears or something like that? And at the end of the finale when Cas disappeared on Dean, it was actually just a hallucination caused by Dean’s fear of abandonment?