It was a cold winters morning the first time it happened. Dean was standing by the window, hot chocolate in hand, when Cas came up behind him and hugged him. Dean was so startled by the sudden contact that he dropped his drink. In the seconds that followed, Dean just stared down at the hot chocolate seeping into his slippers.
"Fucking hell, Cas! What the fuck are you doing?" The warmth of the angel's arms around him made Dean feel safe for the first time in years and it scared the shit out of him. And what was the Winchester way of dealing with something that scared him? Anger, of course. But Cas didn't even flinch. In fact, his arms tightened around Dean's waist.
Dean squirmed, trying to free himself. "Dammit, Cas! The hot chocolate is burning my feet!" he complained. After a seconds hesitation, Cas relented and Dean ran straight to his room and shut the door. He sunk down to the floor, trying to process what just happened.

Cas stared after Dean, trying to process why he'd reacted how he did. He was only trying to offer Dean some comfort, could sense how lonely he felt. But Cas had known the hunter long enough to figure out why he'd been so angry. Anger is a base reaction to fear. And anyone knew that Dean had a fear of closeness. For good reason, too. Being a hunter isn't easy, and Dean had watched his fair share of loved ones die.
After coming to the conclusion that Dean reacted out of fear of emotional attachment, Cas resolved to keep trying. It didn't take an idiot to see that Dean really deserved someone to be close with, and Cas was going to be that someone. It was the least he could do. So he persisted, hugging Dean whenever the chance presented itself. He didn't get as mad as the first time it happened, but he also didn't hug back. After the first week, Dean stopped trying to squirm free, and settled for just standing there until Cas finally let go.
Cas couldn't stop smiling the day Dean finally hugged back. It was a loose, resigned kind of hug, but it meant the world to the angel. It meant that Dean had finally let him in, accepted his love. That word caught Cas, as he thought about the sense in which he loved Dean. It had always been platonic between the two, but now that Dean had let him in, maybe there was room for something more….