As a writer, I find myself having to try to explain my mind quite often. That's a tall order because my mind, and my writing process, is hard to put into a nice little box. But since I hear readers asking me, and other authors, this question all the time, I thought I would touch upon things from my perspective.
First, for non-writers to get a grip on how this author's mind works, you have to realize that I am a high priestess in the court of organic writers. I do not plot out books. If you're looking for extensive plot skeletons go peek into someone else's writerly closet. When an idea appears to me, it generally is a short burst of inspiration. A flash is the best way to describe it. In that flash will be the nugget of the story, a brief glimpse of the leading couple, and perhaps the overlying theme.
That is it.
It then falls to me to gather the flash into a jar, sit it on a shelf, and study it. Sometimes the light withers and goes out. Then I dump the ashes of that dead idea out and put the empty jar back on my mental shelf. If the light continues to glow, then I'll examine the idea in more detail. I'll sketch out character bios, and perhaps fiddle with a title. Then I sit down and I start writing. As I work through the first chapter, I begin to know the leading man or lady better. They slither into my brain and there they stay until the book is done. This is why I can only work on one novel/novella at a time. Being possessed by a sarcastic ass such as Victor Kalinski kind of drowns out any other voices.
Since we now know that I'm an organic writer all the way to my toes, we also must see that there is no set word count for any of my books. I will aim for 30-35 K for a novella and 50-55 K for a novel. Sometimes those novellas come in at 20 K when the ending is called for. I do not know ahead of time what the ending will be or when it will come. It just is there. I can feel it. The light in the jar begins to flicker and I know that the story, as it has played out in my mind, is nearly completed.
This is why a book, no matter how much longer reader's wish it was, can never be longer than it is. The story has been told as far as my creative mind is concerned. Sure, I could pad things up. I could fill in with fluff but I'm not going to do that. I greatly disliked having to make chapters a certain length for an old house of mine. When the chapter is done, it's done. Don't make me add stuff just to reach a number. That's cheating the reader of the words that I felt deeply. Do you really want ten thousand shallow words just to make a set page goal or word count? Me either.
That's why my books end when they do. If they are too short, I do apologize for that, but I do not apologize for not writing empty words to reach a number. Creativity is not something that can be automated or controlled. It must be allowed to run freely, touching the jars of inspiration and setting them aglow.