There's a certain pattern to life and death. Dean has known from a young age that when life begins, death must follow, whether it comes within a few days or many decades. Growing up, he had never been religious; he hadn't even believed in angels until Cas walked into his life, so the concept of anything coming after death included only the comparatively few cases of people becoming ghosts. For most people, life was simple: you lived and then you died. Dean had never expected his life to be anything different. Now, after everything he's been through in the past few years, he knows that there are at least three steps in humanity's life pattern: life, death, and Life 2.0 in either Heaven or Hell.
That's for the average person. Dean's different. Him and Sam and Cas – they've broken the pattern so many times that it makes Dean nauseous if he thinks about it too long. The three of them have lived and died, then lived and died, more times than Dean cares to remember. They seem to be caught repeating the first two steps in an unending cycle, dying only to be brought back to life so they can die again at some point in the future.
Some people would probably say they were lucky. No matter what happened, no matter how many times he lost Sam or Cas, or they lost him, they would always be returned to each other. And who wouldn't want a second, third, or fourth chance at living?
Dean thinks that's a load of crap. Not that he's not grateful to have his family live, but it's the most painful and terrifying routine he's ever been trapped in. Dean lives every near-death moment with a terror in his heart that he would never confess to anyone. He knows that the next time he has to hold Sammy bleeding in his arms might be the day when he doesn't get his brother back. The next time he sees the life threaten to fade from Castiel's eyes might be the day when the angel's wings are permanently burnt into the ground. One day there will be no more miraculous resurrections, and Dean will lose his brother or his best friend for good. Someday he'll be the one to run out of second chances, and Sam and Cas will be left to burn his bones, knowing that their luck, such as it was, had finally run out.
So Dean fights to keep that moment from happening. Fights to keep them all alive and keep death from taking one of them permanently. It doesn't matter if the cycle wears Dean down a little bit more each time one of them is lost and returned, or if they don't always come back unbroken. They're still together. Dean fights the fear, and makes jokes about their brushes with death. He hides his nightmares from Sam, and ignores the soul-searching looks Cas sends his way. He keeps an eye on the two of them and tries not to be too obvious about it, and if the moment comes when either Cas or Sam looks like they might not escape a perilous blow, he will put himself in between them and death because God forbid that he lets either of them die. Not when he knows that he might not get them back. That day is coming; he can feel it. He doesn't know when or where or how it will happen, but when one of them finally runs out of chances, Dean will do everything he can to make sure that he's the one who falls so that his family can keep on living.
