People do things for a reason. Sometimes they fool themselves as to the why, but something motivates people to their actions. The characters if a novel also have to have a reason to act the way they do. Something motivates them.
As often as not and usually in my novels, the hero is motivated by his values. He decides something must be done, a goal reached, on what he believes from his values. He might believe something not obviously true to the rest of the world or his family and friends but completely true to him. His values may lead him to fight an unwinable war or attempt great feats of strength or bravery. Often he desires something abstract that he can't even pinpoint to himself, such as redemption or revenge.
The hero should certainly face many obstacles between him and his goal, but he never gives up though his despair may lead him astray or make him hesitate. The attempt to complete the thing that will help him reach his goal drives the story. Perhaps he wants peace for his world so he must defeat the demonic race over running his people. Many YA dystopian novels have survival as the the hero's only goal although it's usually survival for the hero's family also. That's a pretty strong motivation to propel the hero through a great story.
What's your favorite hero motivation? Have you read novels where the motivation doesn't seem strong enough to drive the hero and story forward?