This wasn't the first time Castiel had a difficult decision to face, but it was getting harder and harder to deny anything to Dean. Standing there looking at Dean's pleading face, willing him to understand he was doing this FOR Dean. Trying to protect him, keep him safe.
As Dean turned his back on him something inside Castiel twisted, and he knew that this was the moment, choose Heaven, or Dean.
Maybe it wasn't that difficult after all.