Plato by way of Aristophanes (Ten Years)

Once upon a time, people were not born separate from each other, they were born entwined, kind of coupled with each other. So there were boys attached to boys, and there were girls attached to girls, and of course, boys and girls together, in a wonderfully intimate ball. And back then we had eight limbs: there were four on top, four on the bottom, and you didn't have to walk if you didn't want to, you could roll. And roll we did. 

We rolled backwards and we rolled forwards achieving fantastic speeds, that gave us a kind of courage, and the courage swelled to pride, and the pride became arrogance. And then we decided that we were greater than the gods and we tried to roll up the heaven and take over heaven, and the gods, alarmed, struck back. And Zeus, in his fury, hurled down lightning bolts, and struck everyone in two, into perfect halves. 

So all of a sudden couples who have been warm and tight together were now detached, and alone, and lost, and desperate, and losing the will to live. And the gods, seeing what they've done, worried that humans might not survive or even multiply again, and of course, they needed humans to give sacrifices and pay attention to them. 

So the gods decided on a few repairs. Instead of heads facing backwards or out they would rotate our heads back forward. They pulled our skin taut and knotted it right here at the belly button. Genitalia too was moved to the front, so if we wanted to, you know, we could. 

And most importantly, they left us with a memory. It was a longing for that original other half of ourselves. The boy or the girl who used to make us whole. And that longing is still so deep in all of us: men for men, women for women, men for women for each other. And it has been the lot of humans ever since to travel the world looking for our other half. 

And when, says Aristophanes, when one of us meets another we recognize each other right away. We just know this. We're lost in an amazement of love and friendship and intimacy. We won't get out of each other's sight even for a moment. 

These are people he says, who passed their whole lives together and yet if you ask them, they could not explain what they desire of each other. They just do.