Sam looks at himself in the mirror. For the first time in a long time, he looks at his naked body and likes what he sees. His scars. He touches them, counting them, remembering the story each one holds. He has never liked his scars before.
Before. Before Jess and dad. Before Castiel had come into their lives. Before, when demons were just demons, and not fallen angels. Hell, before demons even existed for them. Before he got it into his head to go to college, to try for a normal life. Before anyone he had loved had gone to Hell. Before time travel had been within the realm of possibility. Before the apocalypse and angelic wars.
But now, as he admires them in the dim, flickering lights of the latest sleazy motel, Sam can see the beauty in them.
Them something even more beautiful comes into view. He turns and faces his brother, for once not ashamed of his nakedness. "I like your scars, Sammy, they make you look attainable. Like you're real, and I might actually be able to deserve you." Dean walked over as he said this, and kissed the many overlapping scars above Sam's heart.
"Really? 'Cause I was just thinking that I like my scars too, because they might mean I am able to deserve you, that maybe they show I'm a fighter, like you, that I've done some good in this world."
That night the boys marked each other. They left no permanent scars, just reminders that they deserved each other. The shallow, bloody lines Sam's too-long fingernails left on Dean's back. The wound on Sam's neck, where Dean had bitten just a little too hard, and his teeth had pierced the soft flesh. Bruises on both their hips where fingers had gripped too tightly, but not tight enough.
The marks they left that night would fade quickly from their flesh, but would remain forever imprinted on their souls. They were soul scars, scars that would bind them together for eternity.