I do not believe there is a generic answer to this question, that the answer can be as varied as individuals, those there's probably some common threads that run through all of our tapestries?
Opposites are all around us in nature, and i think we are a part of nature. At a ‘basic’ level are positively charged, negatively charged and neutral ion, that attract and form compounds (relationship at a base level?) and some have ‘chemistry’ and form strong bonds (the origins of “bondage?” lol).
It seems, as humans, we are a lot more complex than ‘simple’ ions, but we are composed of them, so i think an argument could be made that Tops, Dom's, bottoms, subs, versatile (neutral), and everything in between on the infinite spectrum, are ‘natural.’ i believe the age old question of “nature vs nurture” is easily answered: “both." i do not see that anyone is devoid of the nature of inheritance or the natural influence of nurture.
i can retrospectively trace being “gay” back to about age 7, and really before. Looking back, i can see all sorts of things about me that were bottom, sub and gay. i grew up in a conservative religious culture that considers “gay” as “broken, deviant, sinful, rebellious, etc..” Also, prior to 1973, homosexuality was considered a psychological ‘sickness’ by the medical field.
Growing up in that kind of society is what conditioned a lot of gays to build the proverbial “closet.” my point here is, i learned how to hide who and how i am in order to survive. i also believed my conditioning and tried to change and become a hetero top dom (i.e., i married a woman and sexually had to be top and was part of a religious culture that had decided D/s under and over tones). It didn't change me though. i sincerely tried to be what i was ‘supposed’ to be, but being a gay, bottom with some sub thrown in, is and has always been a part of who i am.
i think my experience is common to those who may be different from the culture they grew up in, or live in. i think the dominant culture (not the good kind lol) has also conditioned people to feel it's important to justify who they are vs simply recognizing, accepting and learning how to live with who and how we are. i think that happens when those who dominate culture want to assert themselves as superior and not just different.