She comes from a truly neglectful/abusive situation. I know many people call their horses 'rescues' because they weren't in the PERFECT situation when the person bought them, but Kenzie (my filly) legitimately was abused. She was at half the body weight she should have been, had thrush, rain rot, worms and a severe illness that nearly killed her. She lived alone with no shelter, no food, and a muddy piece of land as her home. Later her ear was severed purposely by an angry person who was trying to get revenge on her then-owner.
Most people's reaction to a horse like her, once bringing her home, would be to coddle her. Give her hugs and kisses, forgive normally corrected behavior because 'she couldn't help it', let her walk all over them. I did not. When I first started rehabbing her, even when she still had pressure sores from lying on her death bed for 5 days, when she was barely strong enough to walk, and when every bone in her body was sticking out, I demanded that she respect my space and I would respect hers. She was expected to walk quietly with me wherever I asked her to, and to hold still when I doctored her. When she started feeling good and tried to paw at her food in excitement, I quickly and sharply reprimanded her. Would some people say I was harsh? I don't know. Maybe. But was I fair? Absolutely. I gave her a new lease on life, and all I expected from her was to behave civilly, stand still while I took care of her, and to not get in my space. If she did, it was one sharp smack, yank, or yell...whichever got the point across, then it was back to normal. It only took her once or twice to learn exactly what I wanted and it made her a better horse. She's polite and obedient, and looks to me when she's afraid without trying to trample me. And she obviously doesn't hate me too much considering that she comes running up to me when she sees me approaching her paddock, no matter what she's doing.
Yes, positive reinforcement can be good, but only to a certain extent and only in some situations. And the fact is that a horse who has been taught only by positive reinforcement is never going to learn as quickly or with as much refinement as one who has been fairly taught by pressure and release. It will take a horse, dog, or child a lot longer to realize that he gets a cookie or a pat every time he does x but not when he does y and sometimes if he does z, than to realize that x causes slight discomfort and y means he can continue going forward every single time.