Disclaimers: I didn't own anything about Supernatural yesterday and I still don't.
Spoilers: A few references here and there, but that's pretty much it.
Summary: John watches as his boys grow. (this has to be probably the most original summary I've ever wrote)

One Way Ticket

Beginning

Dean holds on to Sammy like he's everything he's ever had. He soothes his cries and returns his smiles like it's the only thing keeping him together. He blows raspberry kisses into the baby's cheeks and Sammy laughs, so bright that for a life-saving moment, everything seems to be right again.
Until Dean looks up at him, and John wishes it wasn't with a broken gaze that matches his own haunted heart.

Childhood

Sam asks a lot of questions. He fires them off like rock salt on a poltergeist, and both Dean and him can barely keep up.
Sam's questions are weird.
With Dean, it's always been, "How do you kill a werewolf?" or "Does iron repel all kinds of spirits?".
With Sam, it's, "What's the highest mountain on Earth?" and "Where do babies come from?".
Prohibitions for Dean always brought questions such as, "Why can't I use this spell, Dad?".
With Sam it's, "Why can't I play soccer, Dad?".
It's normal stuff, and John has forgot about that a long time ago.
Luckily, Dean hasn't, and he unfailingly manages to sate his brother's ever-present curiosity.
Until one day Sam looks at the both of them and asks, "Why can't we be normal, Dad?".
John doesn't have an answer, and for the first time, neither has Dean.
It occurs to him that Dean's never wanted to know why the sky is blue. It's the first question Sam's asked, and that's when John realizes that maybe, just maybe, Dean's questions are the weird ones.

Teenage Years

Dean trains like he's going to have to fight off a demon as soon as he's done; sometimes it's like the monster's already there. There's an eagerness pumping through his eldest's veins as he runs and fights and shoots that scares John more than any creature he's ever come across.
He doesn't know what Dean's fighting for. He knows it's not obsession, and it fills him with relief, having somehow miraculously succeeded at not teaching his boys about revenge.
But Dean's grown up too fast. And when his son is sixteen and they watch the remains of his first kill burn into the night, he takes a good look at him. He watches the flames reflected into the green irises, and as the last shreds of innocence fade away, John hates himself.
Sam trains because he has to, even though he doesn't understand why. He runs and fights and shoots because his father says so, and he has no other choice but to obey.
He can tell the kid can't wait for this to be over, and it kills him that it's never going to be.
When Sam tells him about Stanford, John's not really surprised, because he's seen it in his eyes and heard it in his shouts as they fought in the past months, that Sam is already gone.

Adulthood

They're grown into men John doesn't know anymore.
Sam's proud eyes are now as haunted as his own, and he dies a little every time he looks into them.
They still fight, they still butt heads. Sam still fires accusations at him. But it's habit now.
The fire's almost gone from his youngest, and secretly John looks to Dean to sparkle it back up.
But Dean can't. He's still fighting, teeth and nails. But he's hanging on by a thread, and he looks as tired as he's ever seen him. He strays to keep the jaws of the hunt from closing around both his father and brother, and swallow them whole.
It's breaking him, and John breaks with him, because neither him nor Sam are strong enough to lift off the burden from Dean's shoulders.

End

John has always thought that he lost his way after Mary's death. He knows now, that in truth there never was any way to find. Because when he opens his eyes to the darkness, he's purchased a one way ticket.

FIN.