LOL! You might as well ask why the sky is blue and the grass is green. Some things, like submissiveness, are inherent personality traits. For other people, dominance comes naturally. There is a myriad of people for whom neither trait is predominant at all times and who choose to embrace one or the other or both to please their partner or to escape from reality for a bit. In others, the trait is dormant until the right person comes along.

I'll leave now that the water is even muddier than before. *grins*