I've never started a horse completely from square one, but I'm working on my current horse kind of from square two. When I bought him, he knew how to stand for tacking up and for mounting, and not to be afraid of a rider, and not to buck with the intent of dislodging the rider. I took it from there, and after a summer of weather permitting daily rides, he now has a good steering wheel, brakes, and is starting to understand "gears" within gaits, rhythm, and relaxation.
I just like the training process, and enjoy taking a clean slate and trying to turn it into a reliable, mannerly saddle horse. My dream is to someday actually saddle-break a baby, but I don't know if I'm "nervy" enough to step in the stirrup for the first ride. And, I suppose if you start with a clean slate, there's less "fixing" to do before you and the horse can progress.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown