Castiel remembers that day so vividly, so clearly, that it pains him with every twitch of his feathers. Thinking about it causes him great agony to pierce his Grace, to stab at his human heart. He remembers how he felt his vessel's heartbeat quickened, but he hadn't wasted any time wondering how and why it did so. All he did was focus his attention solely onto his human. Onto his hunter, his charge, his Dean.
"Promise me, Cas," he had whispered. "Promise me you'll always come back?"
"Of course, Dean," the angel had murmured. "I promise."
Castiel had taken flight, spreading his magnificent wings wide as he flew through the air, feeling the world pass by in a matter of seconds, yet go so slowly below on Earth. He watched all of the colors enter and exit his vision. Castiel tried but failed to ignore the confusion and regret filling his troubled Grace. He inhaled a deep, cooling breath even though it was unnecessary for an angel to breathe, and he let the soothing wind calm his ruffled feathers.
Now he flew through the air, trying to keep himself together as he stayed to his angel duties. I will be brave, he declared, love and affection washing through him as he went about his duties as an angel of the Lord, following and obeying his orders while taking flight again and again. He clenched his fists in determination, forcing himself to stay strong. It quickly disappeared as he felt his Grace waver, and his wings abruptly grew stiff and refused to work properly.
Panic filled him as he suddenly felt his body begin to drop, and his eyes widened in fear as he started to plummet to the Earth. He frantically pulled at his Grace, flapping his wings in terror until they slowly began to work again.
Castiel hurriedly landed onto an empty field somewhere in Italy, quickly searching out to check on his human. He had apparently begun a job, by himself down in central Kansas. Dean stood in front of the door he was preparing to break down, his shotgun held firmly in his hands.
A smile spread across Castiel's lips, and he suddenly didn't care about his stupid orders. He couldn't be away from his hunter for so long, and it's been so long. He took a step forward in the empty field, feeling his wings extend as the doubt dispersed from within him, and he took flight.
"Cas?" Dean breathed, relief and puzzlement echoing in the human's voice. "It's felt like years since I last saw you." Castiel smiled, taking the eldest Winchester's hand.
"Forgive me," he murmured. "I never meant to cause you any pain."
A smile graced Dean's lips as he leaned forward, gently nuzzling their foreheads together. "I have died every day, waiting for you."
Cas shut his eyes, tightening his hold on his human. "Darling, don't be afraid. I have loved you for a thousand years." Dean chuckled lightly, affection reflecting in his bright, emerald eyes.
"I'll love you for a thousand more."