There were many unspoken rules for living and surviving under the Impala's roof, which really all added up to not getting your head torn off of your spinal column by your brother. The demons and monsters were easy. Brothers were significantly harder.
The rules had been developed over the years, starting way back when they were both sitting in the back seat while their father drove the Impala and they drove each other crazy. The rules had only grown more numerous and more vital as they had aged, and especially once they started hunting together. They were paramount after John's death.
Of course, they covered the necessary trivial issues that would inevitably break the camel's back, like 'no smacking gum,' 'driver chooses the music, passenger chooses the food,' and 'no discussing literary references.' They never really had to lay down the rules, but they were understood without speaking of them. Any infraction would surely result in shouting or possibly dangerous pranks the next morning.
There were also the heavier rules, most of them really only necessary to keep Sam in line, especially the more they found out about their past. 'Don't talk about dead family,' 'don't talk about dead friends,' 'don't talk about dying,' and every other combination of death and people that Sam and his overactive imagination and guilt complex could come up with. On the occasions that Sam broke the rules, Dean would first tell him to shut up and stop being a whiny bitch. Usually that worked, but not always. If he continued on, Dean would have to grip the steering wheel with tight knuckles and thin his lips into a stern line until eventually Sam prattled himself into introspective silence and Dean could make an attempt to relax again.
But there were also rules that neither of them broke, holding fast to them with a religious attention that was kept entirely under the surface. These were the rules that were beyond unspoken; they were such that neither brother dared even open his mouth with the words on his lips, even only to let them die before speaking. The other rules were more ordinary, more understandable to strangers, but the rules that they kept quiet were far more important and vital to their survival. Chief among them was the very simple edict, 'don't talk about it in the morning.'
There were times when even the rules of discussing death could be broken, or at least they would tolerate a bending, if it was really necessary, but they never, not ever talked about it in the morning. There was no excuse. They didn't even know what would begin to happen to them if they did. They might not be able to meet each other's eyes. The casual brotherly contact between them, necessary for their job and their sanity, might become suspect and awkward. The long road trips in the Impala, already sometimes painful, would become eternal and endless silences as they both stared through the windshield as the wiper blades whined through the splattered raindrops, no longer sure how to start a real conversation.
Both of them hated the idea that those fears brought, that they were somehow ashamed, but were they looking at anyone else in the same situation, they themselves might be disgusted. But come nightfall, when they had eaten their share of cheap cheeseburgers, drunk their fill of flat beer, and set their weapons in accessible locations around the dirty motel room, they inevitably found each other again. It was only in the dark of night that they kissed and raked their hands through each other's hair with abandon, letting their fingers roam up under dingy plaid shirts and down beneath denim waistbands. It was only behind the shadow of moth-eaten curtains that they could tumble backwards onto stiff blankets and tug off the offending clothing that kept them from exploring the scarred bodies they knew too well. It was only then that they could drag the other down and wear themselves out in sparks of blinding white and stifled groans.
And if, when the tepid sunlight filtered in through the same moth-eaten curtains, they dressed, showered, and gathered their things in silence, it was simply in the rules. Before long, one of them would bring up a job, and then they would be Sam and Dean again, hunting brothers, laughing and shoving at each other's shoulders. They would have more cheap takeout for breakfast before hitting the road, Dean behind the wheel and Sam reading aloud articles from the passenger seat. Even if they both remembered sweat-slick shoulder blades and the taste of second-hand beer on a pair of enticingly chapped lips, they carried on their conversation over the sound of Freddie Mercury and the thrum of the engine. It was just one of the rules.