I think Dean subconsciously (or maybe even consciously) believes that he needs other people a lot more than they need him...unfortunately he's collected a fair amount of proof that this is true. Sam took off to college, Dad disappeared on him, Sam talked about going back to school if they managed to kill the YED (I'll never forget the look on Dean's face when Sam said that -- like someone had kicked him in the guts and run over his dog at the same time). It's little wonder that he didn't think twice about sacrificing himself for Sam. I'm sure he was convinced that Sam would carry on just fine after he was gone and, sadly, that's exactly what happened, so Dean now has even more proof that he's dispensable to those he cares about. :(