"I think I'm gonna finish season 3 tonight, but I feel like I've been spoiled. I mean, the show has more than 3 seasons, so obviously Dean can't die." I thought the same thing when I started watching supernatural a while back!
I have come to the conclusion that Mark is either ACTUTUALLY Lucifer, or working for him. He's made it so we all adore/miss/sympathize with Satan to the point of insanity. Or maybe he's just a hell of a method actor (see what I did there?