Yes, I understand that I am dashingly handsome, rich, and a fierce fighter, bent on protection of the weak and striking fear into the hearts of the wicked, but what is my motivation? ~Bruce W.
According to some psychologists, everything we do and are and want has its motivation buried deep within our subconcious. I don't care. I don't like horror movies, and I don't need to know why I don't like them; I'm not watching them. What I do need to know, is why my character doesn't like pizza. Or why my character is a loner. Or why my character is greedy.
There are some traits that you can get away with not having a "reason" for, like being shy, or having blue eyes, but the vast majority of characterization must have a motivation. I'm not saying to include that motivation in your story. Not even the reader deserves to know everything about your characters; let them have a little dignity and privacy. You, however, their author, need to know.
One of the characters in my current WIP is the villain. I know, you never saw that coming, did you? Well, while looking over my notes with my wife, she happened to ask why he steals. I was taken by surprise. What do you mean 'why does he steal?'! He's the bad guy; he's greedy and wants some money!
Stop right there. We have both a problem and a solution in that statement. First, 'He's the bad guy' is NEVER your answer for motivation. That's called a Disney Complex (Sorry, Walt.) The early Disney villains were notoriously flat characters. No depth, no change, no motivation. They were just bad. Those aren't bad characters; that's bad writing. Every character must have depth and motivation. Stock characters (the Hero, the Villain) are no longer good enough.
Second, we have something good from that statement: 'He's greedy & he wants some money.' This is by no means good enough, but it is a start. He's greedy = character trait. He wants money. He doesn't want to save his family from poverty. He doesn't have a drug issue. He isn't trying to steal bread to survive. His top-level motivation isn't survival or necessity, it's money. This is important because you will write him differently based on his motivation. He will behave differently based on his motivation. A thief that is trying to survive by stealing apples and bread is a very different character than one who steals for the kicks of getting away with it. For them, the money or goods aren't even that important. Very different than my character, who wants the money itself. Why he wants, not needs, money (greedy little punk) is where he needed work.
On the one hand, I was a little miffed. What do you mean he needs justification for being greedy and wanting money?? Would you like to drive down to a prison and go ask the offenders about their motivation? Then I realized, it doesn't matter. They don't need to know their motivation; they just need to know how many years they have left on their sentence, or how not to get caught alone with Joey the Shank. I, however, am a writer, and I need to know why my characters do what they do. I need to know why the villain wants money. I need to know why the hero wants to save people. I need to know why the girl dies. I need to know why he's an arrogant SOB, why he stays aloof, and why she's suicidal.
UPDATE: Thanks to @LisaGailGreen on Twitter, I ran across a very interesting site this week that plays right into this post on Motivations! The Character Therapist is an actual, licensed therapist that will take your character and psychoanalyze them to break down their motivations. Isn't that awesome?!