I've been asked if I wanted a happy ending before. I politely declined, I didn't know it was that kind of place...
Oh, wait.
It really depends on the kind of mood I'm in. If a really good story has a bittersweet or even a downer ending I'm all for it. But If I've had a not so good day or just aren't feeling it, I'd rather things end on an upbeat note. Despite what Dante said in Clerks, Empire Strikes Back did it right. Yes, Han frozen and Luke down one hand is ending on a down note. But that wasn't the end of the movie. It was a down climax, with a denouement that showed it wasn't over and there was hope still. That's not a bad way to do a bittersweet ending.
Ultimately, art is supposed to make you feel something. And it is the same for really any entertainment medium. And, it doesn't always have to be a positive feeling to have impact. Probably why I liked Dr. Strangelove so much. It ends with the probable destruction of all of mankind... and managed to put a smile on my face doing it. Before I saw that movie I would have guessed that was impossible. Same with something like Bubba Ho-Tep. Despite the goofy premise, the mood of the entire piece is pretty somber with themes of inevitability and finality. And the ending is a real kick in the teeth. But somehow I'm smiling through those broken teeth and bloody gums.