This is how the story will end:
Castiel has known Heaven, but it is in this moment that he will finally understand what drives the human race: he will finally understand how such a short life is worth living and he will finally understand why human beings are so innately driven to survive at all costs. He will realize with shock and awe that being cast from Heaven could never have been a punishment in the face of this breathless, all-consuming glory.
Castiel returns Sam's whispered kiss, and it is the most perfect moment of Castiel's life.
But they aren't there yet.
This is how the story will go:
Castiel will realize he's in love with Sam, rather predictably after a hunt that sends Sam to the emergency room in a screaming ambulance.
Castiel will watch with wide eyes and white knuckles through the rearview mirror while the firemen force lungfuls of oxygen into Sam's semi-conscious mouth. Sam will turn from a sickly grey into a pink-tinged sickly grey, and the monitor attached to him by far too many wires will finally stop its alarm. The medic will get an IV on the second attempt, muttering darkly about volumetric shock and tension pneumothorax, and Sam's eyelids will flutter when the medic stabs him in the chest with a long needle. He will speak underneath the oxygen mask, but no one will stop to listen, and Castiel won't be able to read his lips through the fog of the plastic. This will panic him more than Sam's skin tone; he's seen Sam grey and incoherent with pain, but the thought that he might miss the last pleading of Sam's breathtakingly beautiful being will make Castiel want to push the firemen away and press his ear against Sam's bloodied mouth so he won't miss another word.
Castiel will grip the seatbelt across his chest with both hands instead, and he will desperately pray that those lost words will be the last he ever misses.
They will take Sam straight into surgery.
Castiel will sit stone-still in the waiting room, his gargoyle-rigidity unnerving to everyone but Dean. He will stare at a single bare spot on the carpet while they wait. Dean will semi-surreptitiously drink his entire flask in ten minutes and will stalk with barely-contained violence out to the Impala to grab the full one Sam never uses.
Dean will be halfway through it when Sam is rolled out of the operating room: he will be in one piece and breathing on his own. He will still be out of it, but Dean will absolutely not be red-eyed or press Sam's limp hand to his cheek in the darkness of the ICU.
Dean will not fall asleep with his head on Sam's leg, either.
Castiel will take Sam's other hand and link their fingers together on the bone-white sheets. His heart will stutter when Sam's hand tightens around his for a moment, and Castiel will squeeze back desperately until his knuckles are bloodless and he can feel Sam's steady pulse between his fingers. Sam's eyes will crack open and the edge of his mouth closest to Castiel will curl up in an exhausted near-smile.
It will be Dean's hair that Sam cards his fingers through, but it will be Castiel's eyes that he won't look away from; it will be Castiel's hand he will not let go of; it will be Castiel's fingers that tell Sam how terrified Castiel was for the very first time in his life: how the blood ran cold in his veins, how his heart nearly stopped, how he couldn't breathe, couldn't think, couldn't conceive of continued existence in the face of Sam dying.
Sam will pull Castiel closer with the tiny sliver of strength he has left, and he will move his lips as if speaking, but no sound will come out. Castiel will dip his head and lean close enough that he will feel Sam's soft breath on his cheek, and that will be when Sam tips Castiel's head back with their still-linked hands until their eyes catch: glittering blue meeting sparkling green in the pseudo-darkness, and Sam will only have to lift his chin to press their lips together.
But they aren't quite there, yet, either.
This is how the story starts:
Castiel is as near as a fallen angel can get to panic as he shouts at Dean to call the paramedics. He presses his hands to Sam's blood-soaked side, careful to avoid the protruding edges of broken ribs. He curses himself in every language for ever rebelling against Heaven, because he does not have enough grace to save the only human life that matters to him.