Nature's first green is gold,
Her hardest hue to hold.
"I can't do this alone." Sam's done it before, never wants to do it again, wants to hold onto Dean and never let him go, can't breathe can't speak can't live without him.
Her early leaf's a flower;
But only so an hour.
Was it only hours ago, Dean stood in the sunlight, making jokes. How many different kinds of vampires has Dean invented? How many kinds has he killed before, how many vampires have not killed Dean before and this is the day one of them got lucky?
Then leaf subsides to leaf.
So Eden sank to grief,
Sam holds on, holds on, holds on, till Dean's final breath rattles in his chest, until his heart stops beating under Sam's hand...
Sam can't hold on any longer. He burned in hell and it didn't hurt this bad. He screamed in pain as Lucifer tortured him but it's nothing like the vocal outpouring of grief at the end of his brother's life.
So dawn goes down to day.
Nothing gold can stay.