People have a pretty high tolerance for fiction. It's okay to write about a psychotic killer who carves people up. Many fine stories have been based on this gruesome plot device. But it's not okay to fail to provide a motive for those killings. And the thing is, psychotic killers aren't the only characters who need a motive. All of them do.
If you follow me on Twitter, you may know that I spend a good portion of my day watching Investigation Discovery. I can get away with it by telling people it's research -- I write mystery novels. But when I'm by myself and an interesting case is unfolding, I might find myself doing some good-natured (not crazy) yelling at the television screen. Usually, I'm shouting just one word: why?
To me, that's the most important question in every story. Why is the main character in love with this guy? Why is that villain being so mean? Why is this all happening? It's easy to get caught up in writing vivid action scenes, steamy romance scenes and exciting dialogue...and forget all about including a motive.
Something is driving these characters, and readers want to know what it is. There are lots of different ways to expose motive. Maybe you do it right at the front with an exciting scene. A character who nearly drowns, for example, might spend the rest of the book being terrified of water. The beginning scene that shows that near-drowning explains this fear nicely. Just imagine how the book would read without it. Some writers might wait, and reveal the past drowning more toward the middle of the book.
It doesn't matter how you do it, only that you do. Every character has a motive, and you've got to know what it is so that I, the reader, can know what it is, too. Whether you choose to spell it out plainly or reveal it through a series of scenes, you've got to show the motive behind the characters.
Otherwise, I'm going to end up sitting on my couch, Kindle in hand, screaming why at your book. And I promise that, when my neighbors ask, I'll blame the whole thing on you.