The Winchester brothers had lived many live before they died. There was their time as Smith and Wesson. Even though they hadn't technically lived through those lives, Zachariah had still had to plant a lifetimes worth of memories into their brains, so it felt just the same as if they had lived those lives.

And Sam had his adventure in Tuesdays. Sometimes Dean had died early in the day, so the Tuesday was short, but the time loop lasted about a half a year. Dean knew about that part, but Sam had never told him about that final death, the one on Wednesday. After that there was another six months that Sam lived without Dean, before he finally tracked Gabriel down again.

And then there was Hell. For Dean there was 40 years of Hell. 30 years of being tortured and 10 years of torturing. And God only knows how long it was for Sam, and the entire time he was trapped in the box with no one for company except two very pissed off archangels. But Sam was also in the very interesting predicament of being in two places at once. For a good portion of the time his soul was in Hell, Sam's body was up on Earth, so his memories of Hell (once he got them back) were overlapped with the memories that belonged to him-but-not-him.

And then there was the post-apocalypse life they lived through when Balthazar un-sunk the Titanic. Now they both had memories overlaid with other memories.

And even without the big ones, there were time lapses that only lasted a few days, like their trip to Heaven, or Dean's glimpse of the world post-apocalypse.

Overall, by the time they were well and truly dead, the Winchester brothers had probably live roughly four lives each.

So the way they discovered each other was more about survival than it was about passion, lust, or even love. They had loved each other more than life itself even before their relationship became physical and sexual. But the sex allowed them to forget; to not remember any of their many lives. It gave them a small space of time where there was no time, no before and no after.

Subsequently, when they got to heaven for the final time, the place they came to was timeless. They were simply in the Impala, driving forever down a dusty highway. Occasionally they would stop on the side of the road to learn each other's bodies, but now that they were in heaven the memories of all their lives were faded, manageable. The sex got less and less frequent; it was still nice, but it was no longer a survival technique. They only used it now to remember, not to forget. To remember the way the other felt beneath their fingers, the way they tasted beneath their tongues. They only used it now as a physical expression of their love for one another, and as far as either brother was concerned that was the perfect definition of Heaven.