I don't actually think it's a psychological question of "WHY did you become a submissive" as much as it's a question of "what makes you, the way you are, crave submission?" THAT is an important question to know the answer to, because if you can figure that out, you will figure out what makes you tick & will be better able to find a Dominant that matches what you need & want. Sure, we're all submissive because it's our "nature"--but there is something DIFFERENT that draws us each to submission... I think THAT is more the crux of the question.
There is nothing better then spending some time reflecting on your journey as you progress forward. Why? Because we're creatures of habit, and we also are creatures who learn from our mistakes. So if you can spend some time reflecting, you might just learn something about yourself.
Now, I am not suggesting 10hr a day, 5 days a week sitting & psychoanalyzing yourself... but periodically, it really can be a help.