I always think beginnings of books are the funnest part to write, because every possibility lies before you, and any direction is possible. You're still free to create characters, choose wacky names, invent problems and do research. It's just cool, and so energizing!
Endings, on the other hand . . . Just when should a book end?
Thriller writers say the story stops the moment the tension is released. Action, explosion, kidnap victim recovered, and boom--the story's done. I read instructions once on how to write a thriller, and the law seemed to be that when the story is finished, you should get outtathere.
Mysteries seem to follow the same template. I always love Agatha Christie's ending to one of her murder-and-mayhem novels: "You know what we need, dear? A nice cup of tea." The books don't meander on, exploring where we're all going to go now that the excitement is over. We don't know what happens next.
For my own taste, I like a little coda at the end of a book. I like to know how these characters I'm so attached to will recover from whatever stresses the author placed on them. I want a clue as to how they will go on, now that the crisis is past. Will they stay together? Bid each other a teary farewell? Start a new life in a new place? There will be new complications and challenges in characters' lives, and I love to have a hint of that. I like to think, when I put down a book, that the characters in it go on living even though I can't read about them anymore. That little extra chapter, or epilogue, implies to me that the story continues, even though I'm not there to watch it.
And if I care that the story continues, then the writer has done her job.