Motive is a very powerful tool. It can be the difference between the protag and the antag: both working toward the same goal, but with different motives in mind.
What is a motive?
Simply put, it's what drives the characters to act a certain way, to think a certain way, to do certain things. An incentive, if you will. It's the drive that propels the character to achieve the ends to their means.
How important is motive?
This really depends on what kind of story you're trying to tell. Simply having a motive isn't enough to drive a story. It must be tied into the characters as a whole. It must have reason and must have enough importance that we can clearly see why the characters would act or react in such a way that makes the motive identifiable.
Some of my favorite books have some of the best motives that truly drive the characters. And because I'm such a fanatic, I'm going for the obvious - HARRY POTTER. Never have I read such a series that showcases the motives of each character so well, yet intertwines them all together to create a seamless story. The motives of the protag and the antag are so deeply rooted in every action, in every word, in every expression. The readers feel the drive and can either empathize, sympathize, or antipathize with the characters and their journey to the story's eventual end.
How important do you think motive is to driving character and story development?