Castiel has always had difficulty looking directly at Dean Winchester.
Well, truthfully, ever since the incident with Pamela Barnes, he's careful not to meet the eyes of any human, not for long. But in the case of Dean Winchester, he has more than one reason.
He remembers the first time they met.
Not in this guise, the body that was Jimmy Novak's, lent to him for so long now that it's become second nature—but in mortal form, bound and trapped in heavy, helpless, fragile flesh, he saw that face for the first time over a decade ago.
Dean wouldn't remember him. He wasn't wearing this face; his Grace and his speech were both bound, but they'd left him his eyes, and he could still scream.
One of Alastair's minions had caught him spying on their plans, had hindered him just long enough for the rest of them to surround him and overpower him, and he had awakened within the body of one of their victims, its own native soul flung into the void to make room for "our special guest". They'd known, as did he, that eventually he'd be rescued. But in the meantime, they'd found a use for him: a durable, renewable practice dummy for their promising young apprentice torturer. As each body succumbed, he was torn out of it and shoved into a new one, over and over again. As Dean grew in skill and patience, each one lasted a little longer.
Angels don't have nightmares.
What they do have is complete and utter clarity of recollection, so that every time he sees Dean's eyes turned toward him, the image is overlaid with years and years of other images, the same eyes lit with curiosity, with satisfaction, with a craftsman's patient focus on detail.
And every time he hears Dean's voice, it has an echo, the first words he ever heard that voice speak:
Better you than me.