The marks on Dean's neck; as a young man he wears them with pride, like badges. They are mottled bruises and sometimes little bites in pretty shades of dark purple and blue and rosy red that fade to muted greens and yellows. But no matter the color, they are always the same size and shape of Sammy's perfect mouth. Dean doesn't try to hide them - he cherishes each one as a gift, as remembrances of a moment he and Sammy had alone together.
The marks on Dean's neck; the first time John noticed one he grinned wide and clapped his oldest son on the back, calling Dean a skirt chaser and telling him with unmasked delight in his voice that he was growing up to be just like his old man. Dean never corrected him, only smiled sheepishly and cast his eyes down. No one would understand what he and Sammy shared, least of all their father. And so the hidden truth is theirs and theirs alone.
The marks on Dean's neck; he treasures them like each one is a promise from Sammy, reverently tracing them with his fingertips over and over until they've slowly and regretfully disappeared. They are Sammy's claims to his skin. He is Sammy's and Sammy is his and they are all they need in this world.
The marks on Dean's neck; they sometimes blend with the other bruises he gets during their hunts. It's hard to tell at times where one ends and the others begin. But Dean knows. He knows which ones were from Sammy and which ones were just hazards of the job. He knows because those marks still feel like fire, still warm him straight down to his soul, long after Sammy's mouth has pulled away. The feeling lingers, those solemn promises sucked into his skin, reminding him that he is Sammy's and Sammy is his. It's the only constant he has in his life and the only one he needs.