I suggest the hypothesis that parental love and erotic love are on a continuum and that it is through his early interactions with his parents and other humans that a child lays the foundations for erotic love. In fact, the behaviors engaged in by people in love are for the most part the same as those engaged in by parents and children --- holding hands, kissing, talking baby talk, etc. I think that in large part being in love is an effort to recover (or recapitulate) the relationship that the very young child had with his parents. (I am referring here to the initial stages of being in love, perhaps often better described as infatuation.) If this is in fact the case, though, the question remains: why should this be so strongly tied to erotic attraction?
Of course we are often strongly sexually attracted without being in love. But I don't think it's possible to be infatuated with someone in the hand-holding baby-talking way without a strong sexual attraction. (One can still wonder, though, which is the cause and which the effect.) So what is the difference between people we are in love with and those we are merely attracted to?
Romantic love is, of course, a phenomenon of western society. In my opinion, romantic love is the most prevalent fetish of our culture. However, to the best of my knowledge, erotic infatuation is found in all cultures.
Just as erotic love often seems to involve a sort of age regression, the same seems to be often true in SM play, even when it is not part of the overt theme. I have been told this by some female dominants and I know that it is true for me. When I am in "submissive space," I go back to a mental state characteristic of my childhood and my voice also regresses in age. Many of the SM themes that have power for me, for instance that of punishment, clearly relate to my childhood. However they reflect not so much my actual childhood, in which my parents were reasonable, loving people who used punishment only reluctantly, but rather some archetypical harsh childhood, with tyrannically strict parents and teachers.