It didn't really start until Gabriel came back. Well, okay, it started when they first met, mainly because Gabriel had a good taste in humor(though it wasn't really all right that he was killing people, but whatever), but that's beside the point. Sam really, really liked Gabriel. And he was afraid of Dean blowing up if he confessed.
Though, he probably wouldn't actually score Gabriel, unless that porno (That Sam did not finish. Totally.) was just a big show of masculinity. So it didn't really matter. Right? It slipped out anyways, though. It was Christmas time in the bunker, Castiel and Gabriel were off (Gabriel probably getting presents and candy canes and stuff), and Castiel doing God knows what, so it left Dean and Sam alone. They were on the couch, sitting, drinking beer and watching Doctor Who, which Dean of course made snide comments about the entire time, and finally complained when he was drunk enough off of beer and spiked eggnog about 'Why couldn't we watch like, Star Trek, or even Star Wars! Why did we have to watch Doctor Who!?', though they both knew it was because Sam liked it.
After an episode, one of the Christmas Specials, they were talking about which Doctor was the best. "I like Eleven," Sam said, with a small shrug. "He's just… He's kind of like Gabriel sometimes. Okay sense of humor, and while he doesn't do so much of the killing people stuff, he really does have a good sense of humor," he blinked after a moment, realizing what he said, and opened his mouth to speak, but Dean was already talking.
"Dude, I totally know what you mean. Cas is just… Okay, yeah, there's no Doctor that really matches his personality, at least not in the re-vamped series, but…" he trailed off, looking off into space. Sam decided his brother was definitely drunk when he started talking again. "That permanent sex hair and that tie… Just.. Damn…"
"Alright, Dean, no need to undress him with you mind, Jesus," Sam scoffed a bit. "But I was kind of expecting you to blow up about my feelings for Gabriel," he admitted. Dean laughed.
"Sam! Jesus, I've had a fuckin' school girl crush on Cas since I stabbed him… I don't think he appreciated that, though…" Dean laughed some more. Sam raised an eyebrow.
"Mine goes back longer than yours. Way farther back," Sam admitted. "Like, the first time we met Gabriel," he said. Dean scoffed.
"It's not a contest. Gabriel is kinda pudgy anyways. Cas is really fit, and kind of muscular," Dean licked his lips. Sam rolled his eyes.
"I like his pudge. He's probably really cuddly because of it," Sam huffed. Though, it made sense. Dean was more of the butch, manly, not-so-cuddly type, while Sam absolutely loved cuddles and all things furry. Dean huffed at Sam.
"Guess I don't have competition," Dean crossed his arms, looking back to the TV. Sam raised an eyebrow, but shook his head and looked back at the TV also, taking a swig of beer.
Several episodes later, and Dean passing out, there was a flutter of wings (which Sam recognized to be Gabriel), and suddenly he had a lap full of half naked Archangel.
Let's just say Sam had the best Christmas he's had for a while.