At first Dean thought it was odd, the way Cas seemingly obsessed over every flaw of his skin.
How Cas would count every freckle, trace the skin of every birth mark, kiss every scar he'd accumulated over his lifetime of demon hunting. Cas could spend hours mapping out the details of his body, every little oddity that he'd barely even noticed himself.
It was hot and all, but still weird.
One day, in the throes of post-orgasm bliss, Dean caught Cas staring at the dimple in his left cheek. "Man, what is it with you and cataloguing every imperfection of my body?" Cas's eyes shifted to Dean's, piercing him with that sharp blue, as sharp as any blade he'd ever felt. They made his stomach flop and his chest grow tight. He blinked and shifted his eyes down to Cas's jaw, which wasn't much better, but it certainly didn't express as much emotion.
"Every slight alteration of your form from other people's is beautiful. It is an expression of your imperfection, your free will that Eve discovered by biting the apple, what my father himself could not foresee. It is entirely unique to you and only you. It IS you, Dean. Every flaw, every imperfection, makes you perfect."
Dean chuckled, but he was really overwhelmed by everything Cas said. He stated it so matter-of-factly, like they were discussing the weather or a fucking baseball game. Immediately Dean's mind denied all of the things Cas said. He's just being Cas, he can't see what a terrible person Dean is. Dean's crushed by guilt and self-loathing, but he tries his best to hide it from Cas.
Of course, he gets nowhere.
"I know that you don't believe me. I know you don't think you're worth anything, that your life is nothing but mistakes and immoral thoughts and actions. But you're incorrect. You sacrifice yourself for the good of others constantly. You've had to let go of almost everything that makes you happy. You are a selfless, righteous man." Cas kissed him then, pulling him close and holding Dean's face between his hands. Dean closed his eyes, feeling tears pricking at them, and held onto Cas tightly, never wanting to let go. Cas was his anchor, and Dean was drowning. Cas was the oxygen to Dean's deprived lungs. Cas was his home.
They pull away from each other, and Dean presses his forehead to Cas's, breathing heavily against his cheek. "I love you, Cas," he chokes out before he can stop himself. Cas smiles warmly.
"I love you, Dean Winchester."