I have known many horses like your Stormy. I agree it was a matter of gentle training. But often times horses pick their 1 human - other's they'll be good for because they have to, but only one they really want.
I'm going to tell this story I've told so many times before, but when I see them together I am always amazed.
At our horse rescue about 4 years ago a local miniature horse rescue needed to rehome one of their welsh stallions to another rescue because he was too violent for them to handle. We took him as he was small and a quick gelding would do him wonders. So we did. He remained fairly violent after his surgery but just seemed miserable. We called up the other rescue and asked what could have happened to make him seem so miserable? They said "well, he lived with his mom his entire life" 9 years at that point. We said "well can we have her too?" They told us they'd be happy to if they could get her on a trailer.
It took 6 men 5 hours to herd this wild pony onto a trailer. She had lived her entire life wild on their property - no handling what so ever, just bred a few times.
We, the adults at the rescue, couldn't get near her, we had to leave a short lead on her halter to be able to catch her. We'd have to look away and slowly back up toward her. Then she would follow us frightened to wherever we lead her. She was a tiny pony, just barely over 10 hands, and built like a little mustang.
That's about all the work we could ever get with her, we would have to tranque her to get her feet done.
One of the young volunteer at our rescue decided she wanted a pony of her own more than anything in the world and the day she saw this little mare she decided that one was to be hers. So she sat in the pony's stall whenever the pony was in there for quite a few days. She was only 11 at the time. But eventually the pony would come over to her and let her touch her nose. Eventually the little girl could pet the pony alll over. And soon could use brushes of all sorts and finally got the matts out of her mane.
Then one day we asked her to go bring the pony in from the paddock, it was taking her a while so we went to go check. She was out there - not having trouble catching the pony like we would - in fact the opposite. The pony was following her everywhere she went. All around and over the pony followed her. Over the next few months and years they worked together. Now the pony follows her over jumps and tarps and all sorts of 'scary' things.
The little girl is too big to ride the pony now, but she can lean over her and give her big hugs. She's even taught the pony to give her hugs back by wrapping her head around her.
The reason I see this as special is not because it took a very special little girl to be able to do that with this pony, and a very special pony to be able to work with that little girl. I don't think the pony would have come so far with other trainers, adults just horrified her, she needed someone who was eye level at first.
Even still if an adult reaches into her stall to pet her she explodes and gets as far away as she can. The pony will occasionally reach out to pet humans, but if you turn to look at her or reach in to pet her back she'll jump away.
They are special, but I think most horse people have that 1 horse who's like that "just theirs". It's special, but not unique.
Here's some pics of the little girl and her pony :P