A strong motivation comes from the most basic of questions; Why? Why should you write the story? Why is this story unique? Why is the main character on their quest? Just Why?
Personal motivation for creating and crafting a story can come from a variety of sources but understanding what is driving you to write can be far more powerful than simply acknowledging you want to write the next great American novel.
Examining your motivation from a psychological stand point can be quite difficult but drilling down to the core reason can also be empowering. Knowing that you're writing the story because it's an emotional outlet or because you want to know how the lives of the characters will turn out can be the determining factor when a project seems far too daunting.
Just as well is establishing motivation for a character in your story. Far too often a great story can fail because a character's motive is not fully realized. When someone asks why is a character trying to kill another the answer should never be as simple as 'because they don't like them.' Simple motives equate to simple stories and while a simple story can be very successful, no reader likes to get into a plot and receive a childish 'well because...' as an answer.
Finding the core motivation can mean the difference between success and a forgotten venture. Even a brief dive into the psychology of a character or situation can be more beneficial than accepting simple motivations. Dig, dive and understand.