Many novels, especially genre novels, have a built-in motivation. Think: "save the princess" fantasy novels. It's built into the plot. The protagonist wants to save the princess. There's your motivation.Read the whole thing. Bransford also discusses how motivations naturally develop into plot arcs, the need to close such arcs by the end of your narrative and -- in a noteworthy aside -- why capricious, fate-driven stories rarely work. Even Homer gave his man of twists and turns a burning desire to see home, a motivation that has reached down the years and made filmmakers take up the camera and songwriters strum the guitar.
But better yet is a novel where a character wants more than one thing, and these two things are at odds. The main character might want to save the princess, but he might just have his eye on the king's throne as well, so he has to decide by the end of the novel which is more important to him. Better still is a character that wants things that are internally contradictory so that they not only have to battle the exterior obstacles to get what they want, but they have to battle conflicting desires within themselves as well.
Here's a way of illustrating that, Super Mario Bros. style.
Good: plumber wants to save the princess.
Better: plumber wants to save the princess while besting green-clad brother with similar goal
Best: plumber wants to save the princess while besting green-clad brother with similar goal, but although he is brave he is plagued by the creeping sense that the gamer controlling his every move might want him dead
(Picture: CC 2009 by phobus)