Our human race can only achieve happiness if love reaches its conclusion . . .
When a lover of boys, or any other type of person, meets that very person who is his other half, he is overwhelmed, to an amazing extent, with affection, concern and love. The two don’t want to spend any time apart from each other. These are people who live out whole lifetimes together, but still couldn’t say what it is they want from each other. I mean, no one could think that it’s just sexual intercourse they want, and that this is the reason why they find such joy in each other’s company and attach such importance to this. It is clear that each of them has some wish in his mind that he can’t articulate. Instead, like an oracle, he half-grasps what he wants and obscurely hints at it. Imagine that Hephaestus with his tools stood over them while they were lying together and asked: “What is it, humans, that you want from each other?” If they didn’t know, imagine what he asked next: “Is this what you desire, to be together so completely that you’re never apart from each other night and day? If this is what you desire, I’m prepared to fuse and weld you together, so that the two of you become one. Then the two of you would live a shared life, as long as you live, since you are one person; and when you died, you would have a shared death in Hades, as one person instead of two. But see if this is what you long for, and if achieving this state satisfies you.” We know that no one who heard this offer would turn it down and it would become apparent that no one wanted anything else. Everyone would think that what he was hearing now was just what he’d longed for all this time: to come together and be fused with the one he loved and become one instead of two. The reason is that this is our original natural state and we used to be whole creatures: “love” is the name for the desire and pursuit of wholeness.
— Aristophanes
Plato, The Symposium
The Floating Library 
“Apart from this, science has so little to tell us about the origin of sexuality that we can liken the problem to a darkness into which not so much as a ray of a hypothesis has penetrated. In quite a different region, it is true, we do meet with such a hypothesis; but it is of so fantastic a kind – a myth rather than a scientific explanation – that I should not venture to produce it here, were it not that it fulfils precisely the one condition whose fulfilment we desire. For it traces the origin of an instinct to a need to restore an earlier state of things.
What I have in mind is, of course, the theory which Plato put into the mouth of Aristophanes in the Symposium, and which deals not only with the origin of the sexual instinct but also with the most important of its variations in relation to its object. ‘The original human nature was not like the present, but different. In the first place, the sexes were originally three in number, not two as they are now; there was man, woman, and the union of the two. . . .’ Everything about these primaeval men was double: they had four hands and four feet, two faces, two privy parts, and so on. Eventually Zeus decided to cut these men in two, ‘like a sorb-apple which is halved for pickling’. After the division had been made, ‘the two parts of man, each desiring his other half, came together, and threw their arms about one another eager to grow into one’.¹”
Freud, Beyond the Pleasure Principle
Thank you for this, Southern Comfort — the more related quotes the merrier!