As much as we balk at the “happily ever after” ending, the reason we use it still is because people like it. People understand it and it answers all of those nagging questions that make it difficult for people to truly appreciate a story. Now, this flies in the face of something I said earlier about asking questions and mystery boxes so let me add a caveat. A strong ending doesn’t mean that you have to wrap everything up in a nice, neat package. What it means is that however you choose to end your story, your audience leaves with some sense of closure.
The characters journey ends, there is a sense of struggle and something happens to justify the time they spent watching, reading or listening to your story.
Many, many clever people have gone against the grain on those one (take a look at this trailer for Mulholland Drive), but when you do you risk alienating your audience. Definitive endings are like any rule, keep it unless you know why you are choosing to break it.