Did you miss the exchange where they both acknowledged that they wouldn't look for each other? And I'd like to point out that even though Sam begged Dean to do it, Dean wound up being very happy with Lisa and Ben as Sam knew it was Dean's happy ending. Dean wanted to return to that and thought that he would up until the point Lisa and Ben were threatened. Sam has painted his situation as similar (although I suspect it's not). Ideally, Dean should cut him some slack. That said, it's also Dean's character not to do so.