I've often found that a story with a shocking ending or even just a plain unhappy ending leave me with more of a lasting feeling than a story which ends with happiness. For example the film "Remember me" Ends with the main character Tyler dying in the twin towers, I had no idea it was going to happen it just happened right when everything seemed to be building into the generic happy ending. The emotion stayed with me for much longer than most happy endings. Although the emotion was a negative/sad one it was still present and it was very effective. I guess what I'm really asking is, are we ever going to get bored of seeing the man end up with the woman of his dreams every time? Because I can say comfortably from my experience there are far more happy endings than unhappy ones.