User blog comment:ImperiexSeed/Castiel's shifting of Sam's mental problems made no sense!/@comment-4807493-20120405133033

This is just guessing on my part, so feel free to shoot holes in it!

The way I see it, Sam's hallucinations were triggered by not just mental damage, but damage to his soul. I'd say all the stuff that happened to him in the 100 years he was in the cage would have caused signifigant damage. That damage was never really given time to heal, instead it was put behind a wall by Death, and it festered. When it came to the fore, it was too much for Sam to deal with, and that manifested itself as the thing that caused the most damage, Lucifer.

I guessing that when Castiel moved the damage over - and because it was Lucifer who left the most signifigant scarring - it may have left 'echos' behind, so Cas is seeing the same thing. I suppose there's a hole in the sense that Angels don't have a soul in the same sense humans do - its Grace and their core being (I think). I don't think it's the human-mind-is-damaged sort of thing though, more a metaphysical-damage sort of thing.