Why do people do the things they do? In real life it's sometimes impossible to unravel the reason some people do the things they do, but in novels the motivation is essential to the story. It's not good enough to have the hero do good things just because he's the designated "good guy" nor for the villain to do bad things because that's what bad guys do. The motivation for an action needs to be comparable in strength to the act committed.
To be believable, writers need to be students of human nature. They do this by being people watchers, reading news stories, and researching cause and effect through text books, listening to knowledgeable people in various professional capacities, and through drawing on personal experience.
As a journalist, I learned to question Who, What, When, Where, and Why, then found these Ws carry over into the fiction field. It's the Why I'm concerned with today. This past year I've undergone four major surgeries; the last just four weeks ago was the scariest and has left me with the most severe life altering after affects. During my recoveries, along with a lot of physical therapy to learn to walk again and to adjust to becoming a total diabetic, I've done a lot of reading, including a number of books in genres I don't usually read. Along with nearly a hundred books read, most of which I enjoyed, there were some that held little interest for me, three I couldn't force myself to finish, and several that left me wondering what was the motivation behind the actions taken by various characters. There was even one that changed a character's motivation from financial greed to obsession. Actually motivation can change, be enlarged, new factors brought in, but the change needs to be built into the story and made plausible to the reader.
William Faulkner was a master at clarifying motivation. Even his bit part villains rated a back story (not an info dump), leaving the reader with a clear picture of what made that character tick. Faulkner never wrote a dystopian novel, yet strangely two dystopian novels I recently read, A Nothing Named Silas by Steve Westover and The Witnesses by Stephanie Black, reminded me of why I enjoy Faulkner. They both skillfully shared why their characters were in the predicaments they were in, why they continued to fight against the impossible, and why their adversaries were also motivated.
Sometimes people do unexpected awful things seemingly out of nowhere, but a deeper analysis nearly always shows the factors that motivated the action. It's usually easier to understand the protagonist's motivation, but author's often skimp on the other side of the coin. Envy, greed, hate, revenge, sense of inferiority, laziness, political zeal, religious fervor, lies, coverup, jealousy, control, and the list goes on and on for negative behavior. Behind each word is an experience or philosophy that drives the villain and though these motivations are not usually the primary focus of the novel, they clarify the protagonist's dilemma and are important to the story. It's not enough to know what the hero has at stake, when understanding what the villain has at stake clearly ratchets up the suspense and provides a more balanced story. If the motivation is insufficient or weak the story loses credibility.
Those of us who are news junkies and have a preference for printed news find ourselves frustrated with electronic news sources that don't answer all of the Ws. We become even more frustrated with novels that fail to convey why the story matters, why the antagonist does what he does, and why the protagonist cares enough to fight back or escape. Without motivation behind action, there is no story.