Motivation, along with Conflict and Goal are the building blocks of fiction. We already addressed conflict earlier in the month. Let's look at the other two...
Every major character should have a goal, a result or achievement toward which effort is directed. You could say, the goal is the reward or prize the character seeks. Generally, characters should want, no need, something they don't have yet, e.g. getting more money isn't a very interesting goal but getting some money when they have none is more interesting. In fact, the more desperate the character is to achieve the goal, the better. Thus, the goal should be important.
Note, too, that the characters need to act to achieve this goal. Action is very important. Action creates plot.
It is perfectly acceptable for the goals of characters to change during the course of a novel. As a matter of fact, things should get worse for your characters during your novel; in that case, the characters' goals should change. It's usually a good idea for characters to have multiple goals, some small and some large. It's okay if characters do not achieve all their goals as long as they act and something happens as a result. (Sequel, anyone?)
Readers like goals because they want to know what the characters are rooting for; readers want to understand characters. The goal is the "what" of your book.
Motivation is the "why" of your book. Why does the character want to achieve the goals? A motive incites or impels a character to act. Motivations and goals must go hand in hand. Motivations are explained (at least in the author's head) with "because". For example, my character, let's call her Isabella, wants to get off the street because she's cold and hungry. Not bad, but it's better to make the motivations as dramatic as you can. Isabella wants to get off the street because ...a serial killer is targeting homeless girls and she saw him murder her best friend. Much better! So, motivations should be desperate and important, too.
Readers also like motivations because they want to understand the characters. Recall, the ultimate goal is for the reader to have Empathy for the characters.
Authors can step it up a notch by writing both external and internal goals and motivations. For example, Isabella wants to get off the street so she's not brutally murdered by a serial killer (external) AND she wants to reunite with her long-lost dad and earn his respect (internal). In the best books, external and internal goals and motivations are inexorably linked. For example, Isabella wants to get off the street so she's not brutally murdered and she wants to earn the respect of her long-lost dad (a former cop) BY helping the police catch the killer.
Obviously, the final piece of the puzzle is the conflict, the opposition to the characters' goals. Something or someone has to try to stop the character from reaching the goal.
Therein lies the story.