When Dean was fifteen, a medicine man told his father, "You have a good son. Strong and steady, like the earth." John patted Dean's shoulder in agreement, and Dean was proud. Earth meant solid, unwavering. Dean took one look at his father and knew his father was earth too, solid ground on which he could anchor himself.
But Sam was not earth, that much Dean knew. His brother was volatile—a bright flash of warmth in one moment and silent, smouldering embers the next. The wounds he dealt were sharp and searing, and never as solid as a fist. Sam shot out his anger in words, as deadly and ephemeral as fire. But he was also the warmth of his head in Dean's lap, the red of swollen lips and fresh blood, and the source of the coiled heat that simmered in Dean's core and spread though his veins.
Once, as his father drove down a mountain highway, Dean had watched a wildfire devour the entire forest that the road followed along. While Dean's eyes watered at the intensity of the glowing flames, Sam had watched the fire unblinkingly as the orange light danced across his face. Dean was not surprised, then, to find his heart scorched black the day Sam left, taking all traces of warmth with him on the other side of a slammed door. Dean felt as if he were standing in the aftermath of that forest fire years ago, no evidence of the fire left except the death of everything that had been alive.
Yet when Sam came back to him, nothing had changed. There was no need for re-ignition, as Dean had feared. Sam returned just as he was before, as if he had only evaporated away for a while. Something furled awake in Dean's chest again, thirsty but green, and Dean let the familiarity of Sam's soft sleeping presence in the passenger seat wash over him to the beat of the road beneath the Impala. His heart hadn't been burnt to ash after all.
Dean had always imagined Sam's rage to be a violent burst outward, a firestorm meant to scar those close by. But now he watches Sam rage under the relentless machinations of demons and angels and family, and sees a tormented ocean, seized and churned by the winds.
Inevitably, Dean is flayed in the midst of such battles. How could he not be when his edges are his brother's edges? But Sam always calms, always returns, lapping gently at his brother's ravaged shores and polishing the eroded facades. "I'm sorry," Sam would say, "Please forgive me."
Sometimes Dean explodes, molten lava surging from fissures in the depths of his being. Sam, tumultuous Sam, reminds his brother he's not alone, cools the anger, envelopes the despair, and Dean is not only healed, but a part of him is created new. Dean asks, "Do you know how islands are made?" and Sam doesn't tell him. Instead, he says, "It's okay. I forgive you."
They visit the Grand Canyon one day, during a rare time of peace. Dean parks the Impala as close as he can to the edge, then gets out and stands next to Sam. The sun is low in the sky, just at the cusp of the horizon, and his brother's skin glows warm in the light.
"We're looking at the result of seventeen million years of erosion, you know?" says Sam. "Seventeen million years of the river rushing to the watershed and carving the rock on its way."
"Uh huh," says Dean. He looks at Sam, raising an eyebrow. "And?"
Sam turns to face him and grins. "And isn't it amazing?"