I can speak only from my personal experience. I am mostly submissive in BDSM, although sometimes I do like to play dominant role, however I am not interested in switching roles with same partners.
After now 15 years of involvement in BDSM I still cannot fully understand what 'makes' me submissive.
In 'normal' life I am very assertive, have no problem leading teams. In vanilla sex I am mostly active, even like to be rough, hold down my partner, slap her buttocks and so on.
Before I got into BDSM I was quite aware of it, but I was not actively seeking much involvement, and if, I would had rather seen myself as dominant.
As a child I have never been spanked or disciplined in any way resulting in pain, never had enema, never been purposely embarrassed or humiliated.
There is always an issue of nature vs nurture. I believe that nurture can have strong impact on nature, but it cannot change it fully.
Said so, I do not think that sexual preference, especially D/s one is just an straight forward extension of personality. On contrary, it can be complete opposite. According to current understanding how human brain works, we are more in the passenger seat than in the driver. There is a lot that we do without fully continuous control or even with no continuous at all.