Have you ever read a book that was a great story until you got to the end? More than once, I've closed a book and wondered how the ending could be so flat, so trite, so 'let's hurry up and get this over with.' The 'happily ever after' ending doesn't work for all stories either.
A writer has a big challenge in finishing a book or an essay that satisfies all readers. It's probably impossible to do so, but she can at least bring the story or essay to a reasonable conclusion. If the protagonist has surmounted numerous obstacles along life's paths, the reader wants to see her have some kind of peace and/or joy at the end. Reachng that condition, however, doesn't happen in one fell swoop. The writer needs to send small signals throughout the story that this is where we're heading. Not too much to give it away, and not so that the reader will have no reason to keep on with the story.
If the writer ends the story on a down note, lets the heroine wallow in grief or self-pity, her readers may not want to read her next book. After all, we read to be entertained, as well as informed. And we like entertainment with happy endings.
Some writers use the surprise ending. While it may or may not be satisfying, it is usually something the reader will remember. It may leave you liking it or hating it, but you will surely not forget it.
The endings that bother me the most are the ones where the writer doesn't seem to know what to do with the story, so she takes a few paragraphs to finish it up merely to get it over with. It can make a reader feel like the writer got bored with the project and is only out to end it and on to the next project. A reader left with this kind of feeling is also not likely to read that author's next book.
The opening of a story or essay is of major importance. It hooks the reader or it loses them. Endings are every bit as important. It leaves them satisfied or loses them as future readers. Many times I've written a concluding paragraph and realized that it didn't measure up to all that had come before it. So back to work on a rewrite. Don't forget that an ending is the final chance to impress your reader.