One big key in NH (at least in the Parelli program, which is what I follow) is always remembering that the horse is a prey animal, who thinks in a way that makes survival top priority, and that the human is a predator. Horses are born not trusting people, it's in their DNA. Humans look, smell, act and sound like a predator to a horse....we have eyes on the front of our head, not on the sides like a prey animal, we walk upright, we smell like what we eat (meat) and when a predator gets mad we growl (in one way or another). Humans are also very direct-lined in their thinking....we know/see what we want and we go straight for it, and we don't like distractions.
Horses kick and bite each other all the time. But why is it that the lower ranking horse still loves his alpha? Couple reasons. One, the alpha delivered the kick/bite, etc. in a FAIR way and gave the other horse warning. Two, the lower horse's alpha is ANOTHER HORSE. There is no threat involved, there is no danger in the horse's mind, it's just another day in the herd.
Enter the human. If the horse goes to bite, kick, etc. and we deliver a smack, usually followed with a NO! or Stop it! or some other kind of vocal reprimand. To a horse, a prey animal, who thinks NOTHING like a human, this is an aggressive act. We smack and growl. That just proved to the horse that 1) we can't be trusted, that we are indeed the predator he thought we were all along, and 2) that we don't care about his feedback and his thoughts on what was just happeneing. Of course we can't hurt the horse, that's not the issue. The issue is the RELATIONSHIP. Some might say "Horses are so much bigger than us, so you bet I'll act aggressive if he bites." But why should be allow the horse to go there in the first place? Why not listen to him in the beginning and PREVENT the behavior altogether? A horse will only bite, kick, etc. AFTER he gives us warning. He will "turn up the heat" when we don't listen to him, it's like he's saying "Hey! Listen to me! I don't like that!" And then when he's trying to communicate to us, we smack him for it. Not good for the relaitonship. At all.