The horse will generally accept the presence of the human who makes a concerted effort to be non-threatening. Thus it is not so strange that a horse and human can become friends. When your new horse arrives, he has a strong instinct to join a new herd. In the wild the new arrival will usually be challenged by one of the more dominant horses in the herd who keeps him at a distance. Eventually one of the lesser ranked horses will check out the newcomer and they may become sympathetic pairs.
Horses derive social pleasure from such things as ambling around together and reciprocal grooming.
A sympathetic pair engaged in some reciprocal grooming This relationship becomes important when the handler and the new horse explore his new world. I don't mean to say he won't spook when something surprises him, but he will be less likely to lose control of his emotions since his confidence in his human companion and himself will be high. Before long, the horse should be able to read the handler's emotions pretty well. If the handler is unconcerned and curious about how the horse will accept some new thing, the horse will likely be less concerned and curious. Within a couple of days CJ had shown Scooter everything on the ranch.