The angel had been there since his birth, they failed to protect the young one and the best they can do is keep the other one from going the same way.
Dean was two when he first saw the angel, he had woke up to find the man in the trench coat surrounded by a great white bright light and Dean wasn't scared, he smiled and the angel smiled back. Dean knew he was a angel, knew he was a friend, a protector.
The angel never hid from Dean, he made his presence known throughout Dean's childhood. There wasn't a place the angel didn't follow Dean, look over him and keep him safe.
Except no one believed him when they told him about his angel.
Not even his daddy.
"But Daddy look" Dean pointed over to where the angel stood as the angel saw Dean looking at him he gave one of his smiles that Dean had come to love.
"There's nothing there Dean" His dad told him, grabbing hold of his hand and propelling him along. Dean looked back at the angel and waved.
It wasn't until Dean was older that he realized only he could see the angel because Castiel, the angel had told Dean his name over time, was all his.
Castiel, Dean's angel.