AU: Bill Harvelle didn't die in his hunt with John.
When you are fifteen, and the shortest boy in your class (in 7 out of the last 8, anyway), and the eternal new kid, and the insignificant little brother of the mysterious guy who drives all girls mad, you don't exactly get many offers to make out. Your total and utter lack of kissing experience pisses you off enough. It would do even if Dean refrained from constantly asking about the issue just to pick on you.
During the most difficult hunts on summertime, Sam's always been left with either Bobby or Pastor Jim. But Jim's working on some serious case and Bobby and John are mad at each other over something stupid, so this time he stays with the Harvelles for what was supposed to be a couple of days and ended up being more like a week and a half. He sits sulking in the backseat of the Impala, wanting to yell at his dad that Dean used to take care of both of them when he was younger, much younger than Sam is now. Instead, he listens to Dean rambling about how when he's done with high school, he'll join them in every hunt, and they'll never again leave him behind.
He remembers Jo as a bit of a tomboy with a messy ponytail and mud-stained jeans. He doesn't care much for the company, though, because he plans on frowning and sulking as long as he stays there. When the car gets to the roadhouse, Sam gets his rucksack and drags his feet to the front door. Bill is there, ready to go. He winks at Sam and asks him to take care of his girls while he's away. It makes him feel important, even if he can still see Ellen rolling her eyes at her husband's idea. As Jo shows him his room, he notices she's grown up and is now almost as tall as he is. He's tempted to ask her if she's also been kissed, or should he say if she's kissed someone already (she doesn't seem the passive type, not really.) He shakes his head at that thought; Dean would laugh his ass off if Sam discovered a 13-year-old had more experience than him.
The Impala has been gone for hours and Ellen's working on the bar. Jo has suggested two board games, offered to try and find her old Nintendo and gone through her movie collection, but Sam's too busy trying be grumpy and moody enough. She turns off the TV, throwing the remote to the empty sofa and comes out with her last idea for the evening.
"Do you wanna make out?" Jo asks.
Sam mentally searches the room for hidden cameras. When he looks back at Jo, she's staring at him like she thinks he's retarded, or something.
"Do you?", he answers.
Jo rolls her eyes in response. It's not until two years after that that she confesses him those had been her first kisses, too.