What part of LOTR's ending wasn't happy? They get a king back, Gandalf and Galadriel bless everyone's crops, the hobbits get eternal life.

I actually remember the movie trying so hard to fit all the original endings in that it just faded to black at least four times, seemed like it was over, then started right back up again.

Of course, Pellenor Fields did get replaced with a spooky ghost army and the best part of the book (Scouring of the Shire) was left out entirely. Not that it could ever have fit in there.