The first time Sam saw them, he though it was a fluke. They must have had a bet and Dean lost so he had to pay the price with cuddles. Because Sam's older brother was never a cuddler. Sure, there had been small things with Cassie and Lisa, but he was fairly certain there was never anything like this.
One of Dean's hands was tangled in Castiel's hair, the other relaxed against the other man's hip. Cas's arms disappeared behind Dean's back, holding on so tightly, like Dean was the only thing holding him to the earth. His face was tucked into the curve of Dean's neck and they breathed in tandem.
Soft rays of sunlight highlighted their hair and faces and they looked peaceful. Happy.
The second time Sam started to realize that this was different. For one, it was Dean that initiated the contact. Whenever Sam had seen him with girls he would wait until they were basically in his lap before holding their hand or throwing his arm around the shoulder. It just wasn't his style. Until Cas.
They were eating at an old diner and Dean had just ordered two slices of apple pie. Sam had politely declined. They lapsed into a comfortable silence and then something unexpected happened.
Dean reached up and set his arm around Cas's shoulders, pulling him close to his side. The shorter man looked surprised for a moment before relaxing into the touch and smiling up at Dean.
Sam tried not to visibly gag.
By the third time Sam decided Cas was a miracle. He'd thought Dean would never settle down, never retire. He would just keep hunting until something killed him and he stayed down. But being with Cas was relaxing him, the space between hunts growing larger each time. Sam had talked to Garth, who had assured him he had enough hunters and would only call them when they were really needed.
That day there had been a hunt, though, and everyone was exhausted and covered in grave dirt. Dean was limping around on a sprained ankle and Sam let him out right next to the door so he wouldn't have as far to walk. It wasn't until he glanced back in the rearview mirror that he saw Cas had gotten out too and was giving Dean a shoulder to lean on. Dean accepted it gratefully and they hobbled back to the bunker.
Sam saw them a few hours later, Dean sitting at the kitchen table, drinking coffee, Cas standing behind him massaging his shoulders. Their hair was wet and Cas was wearing Dean's shirt which meant… okay, ew, he wasn't going to think about that. Cas leaned down to whisper something in Dean's ear. He smiled and turned to kiss Cas's cheek.
Sam left.
The fourth time Sam realized that no matter how happy he was for his brother, he was unmistakably a third wheel. Dean had excused himself and Cas from the restaurant early, before Sam was finished eating. Sam had grumbled to himself, something about them being akin to sex-crazed teenagers.
After he paid, he was heading back for the car when he saw them. At first he thought they were kissing against the Impala, but as he came closer he saw Dean was cupping Cas's cheeks with his hands as the fallen angel cried.
He only caught a few words, "All my fault. I killed them. It's my fault." And as Dean wiped the tears away with his thumbs, "Not your fault, angel. It was Naomi."
Oh. Sam glanced back at the restaurant. Biggerson's. Damn it, he should've remembered.
He looked back to see his brother holding Cas tight to his chest while the tears soaked his shirt.
The fifth time came the morning after Biggerson's. Sam pushed open the door to their room slowly, almost sure Dean and Cas would still be asleep. They surprised him by being awake, but didn't seem to notice his presence in the doorway.
They were facing each other in the center of the bed, curled out like a pair of parentheses. Staring into each other's eyes, oblivious to the rest of the world, so in love it was almost painful to watch. Almost. Sam would never take pain from his brother's happiness.
The sappiest move he had ever seen Dean make came a few moments later when Dean reached up and started tracing the lines of Cas's face. It was the barest touch of fingertips to skin, soft and gentle. Cas in turn walked his fingers up Dean's arm, touching each one of the freckles.
Dean carded his fingers through Cas's hair, pulling him closer, their lips barely brushing. Sam turned away and shut the door as quietly as he could.
They held hands a lot. Sam grouped this together as the sixth and last time he would be surprised by Dean's ability to show affection. Cas had changed him to someone sweeter (Dean would punch him if he ever said that out loud) and Sam would be lying if he said he didn't like it.
Once they walked to get groceries, coming back with their fingers intertwined on the inside, bags on the outside.
Once by a hospital bed, Cas laughing at something Dean said, despite his broken ribs.
Once in the middle of the "living room" when Cas didn't know how to dance and Dean decided to teach him.
Once when they had yet another Star Wars marathon and let their hands rest between them on the couch cushions.
And once when they fell asleep in backseat of the Impala and Sam glanced back to see Cas's head on Dean's shoulder and their tangled hands resting on Cas's knee.
So by now, Sam Winchester is thoroughly convinced his older brother is in love with a fallen angel. And that's okay. Maybe even more than okay.