Forgive this for mistakes, it was written in a hurry without time for a beta.
Dean's twelve the first time he loses Sam.

He's running around the park, yelling (begging) for Sam to get his ass out here before Dad skins them both. He finds Sam what seems like hours later, staring at a brand new, cherry red bicycle. He's oblivious to the way Dean's panting, doesn't hear the "Watch out for your brother, Dean. He's your responsibility." echoing in Dean's head. Dean rips Sam from his trance, grabbing him by the shoulders and gasps out "Sammy, you can't just take off like that, little brother, you just can't." Sam gives him that damn wide-eyed innocent look and says "OK. Dean," and then starts to list the ways he's figured out to convince Dad to let them get a bike. "O.K." Dean thinks. "O.K."


The next time that it happens, Sam's fourteenth birthday is coming up, and they celebrate it Winchester style by letting Sam go on his first hunt. Of course by this time Sam's all hormones and rebellion, and wants to spend his birthday with his friends instead of being on a black dog hunt. So he and John aren't really speaking to each other, except for the part where John's telling them the plan. "I'll take it from the front and you two circle around the rear. Sam, stay close to your brother." Dean gives him a pat on the shoulder, and says "Don't worry, it'll be easy. I'll buy you a piece of cake after." Except for the part where it's not easy, where there's an extra dog, and he and Sam are running before Dean realizes it. And then Dean hears the screams, and he knows, but he looks around anyway before he takes off to the fading screams and the blood.

They spend Sam's fourteenth birthday in the hospital, not that Sam is awake to know it.


When it happens again, Dean reassures himself that it's not his fault this time. When John said he had to protect Sam, he didn't mean from John. So when he hears, "You leave, you better not come back!" and Sam brushes past him without a backward glance, Dean keeps himself together by telling himself that for once it's not his fault Sam is lost. It's also the first time Dean doesn't go looking for him.


When Dean finally does find his brother, it's only because now Dean's lost John too. But Dean's only with Sam a little while before he loses Sam again, this time to an apartment fire, even though Sam is sitting right next to him.


Since they've started hunting again, it happens more times than Dean likes to think about. Demons, ghosts, vampires, hillbillies, fights, possessions, comas - you name it, it's tried to drag Dean away from his little brother. And each time it happens, Dean tortures himself with "Watch out for Sammy," and "I've got your back," and "If you can't save him, you have to kill him," and "No matter what it takes, I'm gonna save you." Because that, Dean thinks, is what really matters. Dean can accept that sometimes bad things are going to happen to his brother. What Dean can't accept is that he won't always be able to save Sam from those bad things. But in the end he does, and that's all that really matters.


The night before Dean's deal is up, they get spectacularly drunk. Dean wants to be happy and talk about the good old days, but Sam is having none of it. Instead he's looking at Dean like he wants Dean to fix this, and repeating over and over again, "Dean, I'm sorry. I couldn't, I couldn't save you Dean, I'm sorry. It wasn't worth it Dean, I'm sorry. You shouldn't have done it." Dean puts an arm around Sam's shoulder and says "No, Sammy. Don't." Sam doesn't seem to hear, just keeps muttering, "I was supposed to save you. I had to save you." Dean wants to respond, but his eyes are heavy and before he knows it he's asleep.

When he wakes up, his first thought is "Well, today's the day." Only his watch says 12:30 A.M. and he fell asleep around 3:00 in the morning, and sure enough he looks at the date and he's been asleep for almost a day.

He slept through his day of eternal damnation.

About that time he realizes that the hotel room is empty, the Impala is gone, and his last beer smells suspiciously of something other than beer. And then he hears "I had to save you," and "Watch out for your little brother, Dean."