History
Dean was only four years old when his mother was killed, brutally murdered by a demon, and his house burned down. Wracked with grief, his father started out on a years long vendetta and dragged him and his little brother Sam around the country in pursuit. The three traveled all over for years hunting down every hideous monster you've only seen in your worst nightmares. Dean and Sam grew up like soldiers, learning the basics of hand-to-hand combat, weapons training, rites of exorcism, ways to ward off malevolent spirits - you name it, these boys mastered it. Years passed this way - at least until Sam decided he'd had enough and took off. But that didn't last for long. The demon that killed their mom came back and offed Sam's girlfriend, and off the brothers went.
In all their travels, Dean rarely did more than absolutely necessary, but he was ruthless. Saving innocent people was right at the top of his list of priorities, topped only by 1) finding and killing the yellow-eyed demon 2) finding their dad 3) protecting Sam. A simple list, but for Dean it was extremely effective. At least, until they found their dad - and he died. The all-too-brief reunion was cut very short by a demonically induced car crash that left Dean dying and John with little choice but to sell his soul to the one creature he'd spent his life hunting. The Yellow-Eyed Demon brought Dean back from the brink of death, but only at the cost of John's life. John left Dean with this knowledge just before he died - and the knowledge that something horrible was happening to Sam. With his family down to one and the knowledge that he must either save Sam or kill him, Dean's protectiveness of his brother ratcheted up by about a hundred points; he refused to court option two. By the time the final confrontation with the demon rolled around, Dean was ready to sacrifice anything to protect his little brother, psychic messiah of hell or no. And that's exactly what he did. He sold his soul to a lesser demon to buy back his brother's life after Sam was literally backstabbed to death. After all, Sam was Dean's only reason for living - he'd said as much back in Rivergrove.
But the demon who cut him the deal gave him only one year to live. And Dean, who'd normally lived carefully - or carefully enough to keep his and his brother's asses safe, anyway - decided all bets were off and cut loose. "Drink and be merry, for tomorrow we may die" - Dean personifies that philosophy, eating as much as he can, banging any chick who's willing, and living his life like he could die any second. Which, to him, is about right. The job hasn't fallen by the wayside, though. Like a true soldier, Dean is loyal to his duty to the end - only just a hair more reckless about it. Reckless enough to raise his brother's fury. With their positions switched, Sam is now the one racing to save Dean from destruction. But Dean, being Dean, prepares his brother and himself in subtle, cutting ways for his departure - letting Sam tune the Impala, asking for one last Christmas celebration, and always trying to charge off headlong into danger because if he dies, well, he'd die anyway, so who cares? But no matter how hard or how long he tries to convince himself that he's going to hell no matter what, Sam's pleas and his own bottled-up doubts eventually wear him down, making him second guess his willingness to go down in a blaze of glory.
But it's an encounter with a nightmare-stalking Jason wannabe that seals the deal. A little bit of dream root, a little bit of dream walking, and a lot of talking with an alternate, demonic version of himself leads Dean to snapping and taking out all of his rage over his daddy issues and impending death on his alterna-self in one explosive moment of self-realization. This creepy, black-eyed vision of his worst fears and his future is the final kick in the ass Dean needs to start fighting for his life.
But maybe he fights a little too hard. Dean gets a phone call from his dead father, who tells him he knows how to break the demon's deal for his soul. Against all logic and reason, his own and Sam's, Dean races off after this phone call's instructions in one more desperate attempt to prevent his own demise. Too bad for him it's a trap. Too bad for him he happened to be facing in the wrong direction when a grieving father, a nobody, beat down the door and brought a shotgun to bear.
All Dean remembers is hearing the shot when he next wakes up on the floor of Template's cathedral.