My horse has always had a really strong personality. We know the breeder well so we get all the stories. As a foal he would walk beside you and undo your shoelaces with his mouth, when he was 2-3 and they were teaching him to trot in hand, he decided he would get it done quicker and ran off, with the handler still holding on. He is very alpha, and doesn't like being told what to do, but he tries very hard to please and hates being corrected. He has always had an amazing work ethic, and no matter when where or with what, he is with you in any situation, even as a young horse. At his very first show ever, and I had only owned him for 3 weeks he was grand champion and had the high score of the show. He walked off the trailer into new sights, smells and sounds and went "meh, grass!".
I strongly believe that it is the bloodlines. All the horses I have met with his breeding are much the same way. I believe that bloodlines dictate a lot in a horse, just like how people grow up to look and typically be like their parents in how things are organized in the house, to how they drive or even their work habits.