You mean a "happy ending" for the reader, not necessarily for the characters?
This begs the question of "Happy Endings" versus "Satisfying Ending." Aiming for a happy ending may not allow a story its own organic and natural evolution. Whereas a satisfying ending has a sensibility that feels right - in that it would be hard to imagine a different outcome for the character(s) journey. (I am a big fan of endings that are both surprising and inevitable, but think they are a rarity.) Sometimes a sad or tragic ending is the only "right" landing place for our stories. Happy endings are satisfying when they are not forced or contrived - which can be just as challenging to achieve. I say, let the story and the characters show the way.
Yes, Happy readers. But even that is elusive because they so often react as individuals, dang them. And find surprising ways to respond to a story.
I believe a happy ending is one that makes you want to go back to the beginning and read it again.
As a reader: When you hope there will be a next book.As a writer: When you know there will be a next book.