Everything about the horse is linked to its flight response. Crazy as it sounds, even their laziness is related to the flight response. By nature horses are generally lazy, for the sole purpose of preserving energy in case it is needed in flight. In the current trend of natural horsemanship, far too much is sometimes made of the predator-prey relationship, since horses, after all, have been domesticated for thousands of years and don't really think of humans as carnivorous predators. However, it is important to understand that the prey instinct is the origin of the horse's behavior as we know it today and it is what motivates their movement.
Horses are herd animals, again related to prey-dom, meaning their survival is dependent on the herd.
Safety in numbers. Herd behavior is another important motivating factor for a horse and is present in our everyday dealings with horses, more so than is often recognized. Again, every movement a horse makes has meaning and when given a choice, the horse will always move toward the protection of the herd. These are fundamental and deep layers of horse behavior and the subject could fill many volumes, but the one thing we can deal with here, is to develop an understanding of how we can control the movements of a horse in our presence. The first thing to understand is that the horse feels safer when he is moving his feet, and the more nervous or uncertain he gets, the more he wants to move his feet.