Motivation is a key-concept in psychology and in fiction. It is a man’s basic premises and values that form his character and move him to action — and in order to understand a man’s character, it is the motivation behind his actions that we must understand. To know “what makes a man tick,” we must ask: “What is he after?”
To re-create the reality of his characters, to make both their nature and their actions intelligible, it is their motivation that a writer has to reveal. He may do it gradually, revealing it bit by bit, building up the evidence as the story progresses, but at the end of the novel the reader must know why the characters did the things they did.