The first time it happened, Dean didn't think much of it. He brushed it off because everyone dreams of dying, right? The second time was pretty much the same. Sure, this time he woke up in a puddle of his own sweat, but Dean didn't care. Even if he'd died at the hands of himself this time.
It shocked him when the dreams came into his daily life. He remembered the first time it happened, sat staring out of an old diner window, lost in his own thoughts. He knew Sam was speaking, but god that kid was boring sometimes, so he let himself zone out. He didn't even realise, but he was imagining his own funeral. There was no one there except his body lying in the plain casket with slit wrists.
It was only when Sam shouted "Dean, are you even listening man?" that he thought about it properly. Dean was fully aware of how he'd died in that daydream. And he'd never admit it, but it scared him how effortlessly his brain had conjured the image. Moving around all his life meant that he didn't have many friends. But no one was there. Not even Sam. Or his dad. Or Cas.
After that, everything changed. Dean began to imagine his death more often. Involuntarily thinking about overdoses, standing on a ledge, the feeling of adrenaline as a blade rubbed against his skin. He genuinely tried not to, because that's what teenage girls do, not grown hunters god dammit Dean pull yourself together. He could say that to himself a million times but his brain screwed him over each time.
There became a point where he knew he had to accept these feelings. Because he could kill werewolves and poison vampires but Dean couldn't figure out how to kill his own brain for a few hours. And it felt like he'd failed.