I think it is mostly an impulse to create something together. I don’t have a good model for the causal mechanism of how it works, but I think that attraction essentially hints at a specific potential between the two parties. Sex and “baby” is the creation-potential only a tiny fraction of the time.
Here are some of the many alternative creation-potentials, in no particular order. Would love to hear what else y’all notice, as this is not an attempt at an exhaustive list:
A project for you to create together
as small as a party and as big as an organization
An emotional gift one of you has for the other
eg: a piece of advice, sharing of life experience, an introduction to someone important
Similarly, information share—eg: a mutual friend needs to be taken care of
A quality or characteristic for you to integrate/embody more: The classic projection of a “golden shadow”
Eg: “She’s so strong” because I’m not claiming my strength, or “he’s so smart” because I’m not comfortable admitting my own intelligence
A psychodynamic transformation (aka "healing")
Eg: you projecting your mom/dad stuff onto each other can be seen as a beautiful chance to make it conscious so you can be more present and available in all your relationships
A different kind of relationship: Ongoing friendship, mentorship, employment, or some other awesome/potent relationship
A chance to see what you’re avoiding in other relationships. This is the classic where someone cheats rather than deal with stuff at home.
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You meet someone new and feel that sudden frisson of erotic excitement. That’s a crush—must be romantic love, right? We’ve watched it happen so many times in ourselves, our community, our movies and books, that it’s almost taken for granted. But I think as soon as I point out the alternatives, they seem obvious too. I wonder if there are other media and cultural narratives to help support more awareness of the alternative? I feel like this would help people be more open to different kinds of love, different kinds of relationships, and suffer a lot less. I feel like this awareness has done so for me, at least.
Hat Tip to Ken Wilber, where I first heard of using the word “eros” as the creative impulse of the universe driving to more novelty.