Why does a particular horse accept being touched and rubbed all over except for his ears? He hasn't been "eared" or hurt in any way. He has no reason to expect being hurt. He simply has never been desensitized to touch in that "compartment" and now has produced a habitual behavioral response that keeping everyone away from his ears is the thing to do.
Why does a particular horse who is quite comfortable with humans around his head and shoulders shy and threaten to kick when someone approaches his hind end? He can see perfectly well back there and like the human perched on the fence, he surely knows who this person is. He's never been attacked by a human from the rear yet he is instinctively defensive.
Each horse has a different level of innate defensiveness. Some are pretty laid back and accepting while others may range through the spectrum to dangerously reactive. But the fundamental behavioral triggers remain the same throughout all of these animals.
Stress creates defensiveness in the horse. Defensiveness typically strengthens compartmental boundaries. This explains why the laid back horse seems to accept everything while the uptight horse has hundreds of worry points.
There are several approaches that we consider part of the Least Resistance "family" of horse training methods. While the mechanics of some of these approaches may differ greatly, the consistency between these approaches is that when applied correctly they reduce stress in the horse, raise the horse's level of consciousness, diffuse the horse's defensive compartmentation and promote comparative thinking on the part of the horse (what is OK in compartment "A" is therefore OK in the other compartments.)