Cowboys, and horseman from years gone by believed that a horse had to be dominated, to be controlled; and since horses´ use force with one another, these trainers believed that force was necessary with mankind towards horses as well; in order to earn/deserve a horses´ respect. You will note that horses in a herd will sniff nostrils, and "talk" to one another.
If one disagrees with the leadership of another, he will "snort" "squeal" and generally be quite vocal with his disagreement. That is where the conflict ensues between them. Usually the newcomer is the one challenging the hierarchy of the other.
The established horse will usually use a) charging, b) biting, c) kicking to reinforce his position within the herd. It is from this behavioral pattern that "old tyme horsemen" believed that horse´s would only submit, respect, obey if you could prove to the horse that you were more aggressive or dominant. This method worked most of the time, and with most of the trainers. But, what if you were not an aggressive, bold, or dominant personality?
I am sure you can see that you would hate horses for forcing you to behave as they do in order to influence their minds and hearts into believing that you were the one to follow, that they could trust you to take care of them. Thus came development of "Natural Horsemanship".