Waitaminute. We've covered the abuse thing and all, but... what about the non-tramatized, dominant submissive?
I didn't come from the happiest home, but I was neglected more than abused. My childhood wasn't THAT bad, really, and whatever issues I developed because of it have long since been addressed.
But more importantly, I am an INCREDIBLY dominant person in my life in general. I was the manager of a local newspaper. I'm the strong shoulder others rely on. Before I knew my sexuality as well as I do now, I would attract very submissive boyfriends. I am, by all measures, the perfect candidate for a very powerful Domme.
And yet, I am not just submissive, I am a slave. I am an ENTHUSIASTIC slave. Well, for the right person. Very few know about that aspect of me, not because I am ashamed, but because there are only certain people who trigger it in me. I don't have a Dominant side in bed, really. I'm vanilla or submissive. That's it. I can play Dominant for a scene, and I can do it well, but I don't particularly enjoy it. I'm doing it for my partner, not me. So in a sense, I am acting dominant in a submissive way.
I fantasized about acts of submission since childhood, and always resented the submissive men I attracted. I remember, in my mid-early teens, trying to encourage them to take more control. Topping from the bottom. But they wouldn't. It wasn't in them to do it. They were attracted to the General Life me, the dominant personality.
It is only very specific people who notice how submissive I am, and only a tiny percentage of those have the ability to own it. But for the right Dom/me (I'm bi), I am a dedicated sub.
So, what makes me submissive? It's not my childhood. It's not my general take on life. It is a very specific release valve, maybe in response to my generally dominating personality. The strength that allows me to lead so well in my life at large also allows me to complete surrender to another while maintaining my sense of self. I am an intense person, and in love, I will push myself to my limits. That is what makes me submissive.
I am submissive because of my strengths, not my weaknesses.
Basically, we have all different reasons for being submissive. Some are like this, some are like that, so making a generalization is just excluding everyone else.