Yeah, I think saying King never writes a happy ending is a little too pat an answer.
In life, I've only ever seen one truly happy ending. My great grandfather died suddenly and quietly while sitting on the tailgate of his old pick up truck, right next to his, "pride and joy", wonderful strawberry patch. But even that wasn't a truly happy ending because the rest of us were left. In this regard, I think King deals more in realism than people want to give him credit for. There's always hope, but there is also always it's dark twin, dispair. And of course, can it be a true ending if we don't follow all the characters to death? I don't know, endings are really never that great in my opinion.