...Basically, I think there are many depths and layers of the "correct" answer, and we can only begin to dabble in the surface.
Another thing to consider is that a lot of horses prefer women. I imagine for the softer, more nurturing nature of the woman. So perhaps the horse plays a significant role in why women like horses.
Synaptic Self (http://www.amazon.com/Synaptic-Self-How-Brains-Become/dp/0142001783/ref=sr_1_1?ie=UTF8&qid=1296328434&sr=8-1) and Deep Survival (http://www.amazon.com/Deep-Survival-Who-Lives-Dies/dp/0393326152/ref=sr_1_1?s=books&ie=UTF8&qid=1296328652&sr=1-1) are both good books that wrestle with why we do what we do.
My sister and I were raised in a no horse home. She is by nature very emotional and very in touch with her feelings. I'm very goal oriented and not much concerned with feelings. She's a teacher. I was an F-4/F-111 WSO/EWO - NOT a warm and fuzzy profession.
I've always liked horses. She wouldn't care if all the horses died. So my 'relationship' theory dies, when applied to us. I still think it is a useful ROT, and I think it is a valid one sentence explanation for things that go deeper than any man understands. God may know the heart of a man, and the Shadow may know "what evil lurks in the hearts of men", but I know neither.
I will say that my approach to riding differs from my wife and daughters. They like horses that are friendly and reassuring. They find Mia a bit scary and indifferent. I rarely get on Mia's back without an idea of what I want to work on. Maybe it is for Mia, or maybe for me, but I almost always have a goal for our riding. Friendship wasn't a goal. It was something that happened as we worked together.
I think most horses are more in touch with their emotions than humans, and respond well to a human that can give and take at the emotional level. Not all horses are like that, and a dominant, goal-oriented horse Like Mia can become a nightmare for someone who isn't her match. I don't write Mia poetry. I don't go to her corral and read her books. I don't have daydreams about her, and I'm certain she doesn't dream about me.
I dunno. Maybe someone smarter than me can figure it out. For me, I'll have to quote the King and I:
There are times I almost think
I am not sure of what I absolutely know.
Very often find confusion
In conclusion I concluded long ago
In my head are many facts
That, as a student, I have studied to procure,
In my head are many facts..
Of which I wish I was more certain I was sure!
(Spoken): Is a puzzlement!