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It starts slow. Or maybe it starts suddenly, in the bang of barn doors and a shower of sparks. But then it's slow, a sequence of tiny moments that put an ache into each of their chests. Dean understands first, no stranger to the feeling of longing. Cas, new to all of this, takes more time to recognize the yearning he feels. Each believes he is alone in his desire, in his pain, and each strives to hide these feelings from the other. They maintain their fragile equilibrium, unaware of the way they unconsciously match each other move for move.
Sam sees it all. He's the audience to their drawn-out dance as they circle each other in slow motion. He watches Dean snap a sneaky candid of Cas grinning at the screen while they're watching Scooby-Doo in the Dean Cave. Then three days later he sees Cas cop the same move while Dean's peacefully washing dishes after dinner.
It's crazy-making, and sometimes it frustrates him so much he almost vibrates with the drive to interfere. He wants to grab them and shake them by the necks. "How can you be so blind?" he'd yell. "He's in love with you!"
It's not his place, though. They'll get there. What's between them keeps growing, and one day it will bloom.