The way Dean said he knew it had a slight triumphant ring to it and Castiel knew he'd have to elaborate. Dean was his best friend and it was unfair to hide the truth despite the fact it may cost the friendship. The idea of that loss was what held Castiel's tongue. Most of his connections to the human world were through Dean and if that was taken away the loss would be incredible to the Angel. Of course, he'd still have Gabriel and possibly Claire but the rest of his new found family would be gone.
Gratitude for the compliment shone in Cas' eyes even though he didn't fully agree with Dean's assessment. He wasn't going to argue that if Dean had seen other Angels' wings he wouldn't find his as incredible anymore. He shook his head, lowering his gaze to a spot on the bed sheets. "No, I am swift because of what I was created for." He spoke with an anticlimactic hushed voice.
He worked his jaw, clenching it tightly and swallowing, the only sign of his uncertainty. Perhaps he should take the risk and explain. "Wings are... scared." Castiel began, sounding like he was reciting from a textbook but still not able to make eye contact with Dean least he get distracted or chicken out. "They are the only part of our true being that a human can perceive without burning out their eyes. Even then we aren't suppose to expose them, whether that be fully or via shadows." A soft sigh escaped his lips, the memory of their first meeting bubbling to the surface. "Even though I knew it was defying orders I knew the only way you'd believe I was what I claimed was to show you. I don't regret that."
Cas chanced a quick glance toward Dean before looking away and continuing. Fear bubbled inside him, knowing what he revealed could break their friendship, turning it into something ugly and awkward till they end up calling it quits. "As you can imagine, forcibly removing a feather is a big taboo among Angels. But I've already rebelled so what was one more strike against my name?" The question was meant to be rhetorical but it also served as a reminder to himself of what he had given up. "Black feathers are meant to be a sign of protection. I wanted to give you that so that if one day we are separated again you will still have an Angel's protection. Theoretically, anyway."