Weve HAD the war, weve HAD the death and despair, a Movie like this needs a happy ending!
That's the kind of stereo-typical hollywood thinking that marrs these films, in my opinion. It's similar their their introducing the warg attack in the second film, because they think Aragorn needs to be put in jeopardy. It's formulaic.
And the book does have a happy ending, of a sort. And the film's ending seemed interminable, so really wasn't that great.