I would also have a chiropractor out to check him out thoroughly.
Next I would evaluate his feed. I would take him down to just 100% bermuda grass hay, no grain, no treats, no salt blocks, no supplements. Give him as much hay as he wants to eat. Let his body detox for 30 days or more. If his attitude begins to improve, then I'd say he has a food allergy or sensitivity. I would keep him on an all hey diet, and just add a vitamin supplement with a handful of grain or 1lb of a ration balancer (like Purina's Enrich 32 or Triple Crown's 30% supplement). If he loses weight, add Nutrena Empower to his diet, just 1-2 lbs of it (it's a rice bran/flax pellet that is high in fat) or see if you can find rice bran pellets and mix them with some shredded beet pulp (soaked or dry).
Illness, Pain, and Diet all play HUGE roles in horse behavior. Since this is a new behavior, I would look to one of those causes first, before attributing it to training or attitude change.
I also agree, personifying a horse is pure folly. A horse is an animal with lower thinking capabilities. I love mine, baby talk to them, and spoil them, but in no way do I think they feel or "think" like a human (dignity is a human concept). I still train them like a horse, using pressure/release and reward/punishment. They are well behaved and have PLENTY of spirit, lol.