A is for Angel
Dean Winchester is just six months old when he first meets Castiel.
The oldest Winchester was a curious baby. He laid awake for hours at night, simply staring at the ceiling like it was the most fascinating thing in his entire existence. Not a sound left his mouth; he was simply enraptured with that ceiling.
John and Mary were puzzled as to why Dean was awake, but never cried out. Surely he needed a bottle or a diaper change. Why would he be up if he didn't?
Little did they know the reason Dean awoke at night wasn't because he was hungry, but because he saw a beautiful cloud form on his ceiling.
It was every color of the rainbow and probably more that weren't perceptible to normal humans. It shone softly like the moon and glittered like fresh snow. The cloud watched over him every night and didn't leave even after he'd become too exhausted to lie awake.
Six month old Dean felt more comforted by the shiny cloud than he did by his mother's singing and didn't want to lose a single moment of that soul consuming peace.
The centuries old angel knew what he had to do. He must stay with Dean every night, provide him as much comfort as he possibly could because he'd seen the future, seen Dean selflessly save humanity over and over again at a cost to himself.
Castiel knew that this tiny baby would one day grow up to save the world.