There was a time and a place for everything. There was a veritable encyclopedia's worth of unspoken rules of the road, though Dean insisted on relaying the "Driver picks the music…" axiom aloud frequently. But there were some things that superseded even the most stringent Hunter philosophy…because being a younger brother outranked everything else.
It wasn't often that these moods came upon Sam. He was analytical enough to recognize that in their relationship Dean was almost always the one who initiated physical contact, be it a hand on the shoulder, a slap on the back, whatever. So Sam's inclination to start something was rare, but when he did follow through on an impulse like this it always surprised Dean which gave Sam a slight advantage.
Sam just couldn't resist himself. They were sitting side by side on Bobby's couch relaxing after a job well done and not thinking too much about the future. No one was injured or bleeding; everyone was well fed. The sun was low in the sky and there was absolutely no humidity. Everything was perfect.
Sam shifted as if he was going to get up from the couch, but as he moved he scooted closer to Dean. Grasping Dean's right wrist with his right hand, Sam picked up Dean's arm and gave Dean an open-hand slap with his own hand. "Stop hitting yourself, Dean."
Even if Sam had stopped right there, it would've been worth it; the look on Dean's face was priceless. He was caught somewhere between surprise, irritation, anger and payback. While Dean was still processing Sam hit him again. "Why are you hitting yourself?"
Dean glared. Sam grinned.
Dean's arm tensed though he did not pull it out of Sam's grasp. It was a silent signal - try it again. And in the great tradition of younger brothers everywhere, Sam did.
Bobby's cries of "Take it outside, ya idjits!" were drowned out in the crashing of feet and the laughing of brothers.