Who doesn't love a happy ending? When you pick up a book (especially a romance) you expect your main characters to find exactly what they need by the time you read the last page. It gives you hope. Hope that there is still a happy ending for yourself out there.
Happy endings aren't always what they're cracked up to be. If every story ended happily ever after, all of them would lose their appeal. What you want to give your readers is the most memorable ending possible. Sometimes it's happy, sometimes, not so much. Nicholas Sparks is the master of the tragic happy ending. Even when one of the main characters die, you still feel good about reading the story (and lets face it, everyone has read one of his books or seen an adaption).
The main thing about ending your story is making sure all the questions are answered. After all the crap your characters had to muck through, did everything get answered? Live or die, happy or sad, good or evil--do your readers want more without asking why this happened or how it was resolved. If you can answer yes, then you do have a happy ending.
Aside from the story itself, as a writer, you're looking for one more happy ending (and yes, this one does have to be happy, or what the heck has all the work been for?). As the person who wrote every precious word, wracked their brains over the character development, plotline, query letter, synopsis, etc. etc.-- you have to be happy with what you wrote. And that's the only happy ending that really matters.