What is the backbone of Buck? What keeps him motivated the most?

I think Buck's "call of the wild" keeps him motivated. He feels it when he first gets to the North and it grows inside of him. He is subservient to man but the draw towards the natural world and autonomy grows inside him. After pays his last master, Thornton, back by killing the Yeehats, Buck takes his rightful place as alpha dof of the wild.