Angels had been watching over Dean for a long time. Angels woke him up on the night of the fire that killed his mom; Dean felt the heat and smelled the smoke. Angels made him turn back and find his brother after Sam went back to the apartment he shared with Jess; Dean noticed flickering lights. Angels made Sam call Dean for three hours before he was supposed to leave on a bus to San Francisco, convincing him to come back in time to save Dean from the scarecrow; Sam remembered something about pagan gods that Dean probably needed.
But Castiel's first assignment relating to Dean Winchester was in Roy Le Grange's tent.
See, Le Grange wasn't lying when he said the Lord helped him see into people's hearts and choose whom to heal. Although Roy's wife had bound a reaper to her and used it to do her bidding according to her own judgment of good and evil, the angels of Heaven gave Roy his brain tumor that drove her to save her husband. After Sue Ann bound the reaper, Castiel was assigned to Roy to guide him in healing people that were important to the Divine Plan.
People like Dean Winchester.
Castiel stood back by the alter as the crowd shuffled in, waiting for the moment when he recognized the one he was ordered to have healed that day. Not the man on crutches, he would soon meet his new nurse and marry her. Not the woman in the purple jacket, she was here to pray, not be cured. Not Layla, she would soon go to her heaven, which would be wonderful in case you were worried, and her mother would join her after a few decades. But two boys had just taken seats in the second row; one was perfectly healthy, but the other… This was the one. For some reason, this boy was important to his father and Heaven.
Castiel strode to Le Grange and directed his hearing to pick up Dean's comment about looking into people's wallets.
"What's your name, son?"
"Dean."
Castiel whispered into Roy's ear, "This boy, Dean. He has an important purpose. He has a job to do, and it's not finished. Heal him."
The preacher smiled, "Come on up here, Dean."
Roy and his angel, and Sue Ann and her reaper healed Dean that day. Afterwards, the Le Grange family no longer figured into Heaven's plans, and the Winchesters were made to ensure that Sue Ann no longer abused her reaper's powers.
Castiel was called back to Heaven and given a new assignment. Apparently, there was a war coming.
AN: Hey, look! A story that's not Sam-centric! I've been working on some headcanons from the Cage for the past two days but it's been really hard and slow. Lucifer is REALLY hard to write! I thought I'd refocus and rewatch some episodes, but the next one in my queue was "Faith" and this came out instead!