Cas is addicted to Dean’s smiles.
He loves the way his face lights up, the way the corners of his eyes crinkle, the way the years and the life he’s lived lifts, just for a moment. He loves every smile Dean has stored away; he’s categorized them all.
There’s the smile that appears when Dean talks about Sam. It’s small but proud, filled with all the years Dean has spent caring for his brother; sometimes it’s a little bit sad, when Dean remembers the life Sam could have lived; sometimes it’s irritated but fond when he talks about the annoying things Sam does when they don’t have a hunt or the end of the world distracting them.
There’s the smile that’s more of a smirk when he’s right and someone else is wrong, a teasing, arrogant quirk of his mouth. He flashes it around even when he isn’t technically right. Sam tells Cas that it’s because his brother is actually a dick; Cas loves the smile anyway.
Sometimes Dean tries to hide a smile, looking down and away. Cas loves this smile too, because when it breaks through it’s brighter than usual, filled with the kind of joy Dean should have had more of as a child.
Sometimes the smile is brittle, hard, masking a pain too heavy for one man to bear. This smile makes Cas’ heart break a little every time he sees it.
More often than not, the smile Cas sees the most is the fond yet irritated smile Dean shoots at him whenever he misses a joke, or misjudges a social cue. Sometimes it’s a glare instead- but Cas can see the smile anyway, tucked into the corners of his mouth where he thinks Cas can’t see it. Sometimes Cas purposefully acts oddly, just so that he can see that hidden smile. Sometimes, if the situation is right, the smile turns into a full blown laugh, one that moves through Dean’s entire body, and Cas knows he would do anything just to see that laugh again.
And then there’s the smile that light’s Dean’s face when they’re lying clse together on the bed, their limbs entangled and their lips brushing. The smile he can feel in his bones like a soft glow, the one he can’t help returning as they move against each other murmuring nonsense into each other’s skin. This smile belongs to Cas and to Cas alone; when he sees it, he knows that he could never live without it.