Chemistry happens between horse and human just as much as it does between human and human, and it’s just as important.  I don’t know how many horses I’ve ridden that I just didn’t get along with, even if they were docile and well-trained and we performed well together whatever it was we were supposed to do (show, race, ride down the trail).  Sometimes the lack of chemistry has meant that a horse that is a perfect angel for other riders is a little monster for me (and sometimes it’s vice versa).
But… you can learn to get along with any horse, and the more you work with a horse (and the more it costs you), the more you are likely to end up loving that horse in the end.