"Every heart sings a song, incomplete, until another heart whispers back. Those who wish to sing always find a song. At the touch of a lover, everyone becomes a poet."
Plato, the great ancient Greek philosopher, in his Symposium, offered an explanation for love and the desire to be with another person.
He described the original race of men as having four legs, four arms, two faces - double of everything that we have today. He said there were three sexes: "the man was originally the child of the sun, the woman of the earth, and the man-woman of the moon, which is made up of sun and earth."
Mankind was powerful - so powerful that we rebelled against the gods. The gods knew a punishment was due. But they did not want to destroy us, for that would mean they would lose all the nice sacrifices and piety we had to offer. Zeus decided that the best punishment would be to split each and every one of us into two, so that we'd be "diminished in strength and increased in numbers" and thus more profitable to the gods.
Since then, we have all been searching for our other half. The original male "children of the sun" each became two homosexual men. The original female "children of the earth" each became two homosexual women. The original adrogynous "children of the moon" each became one man and one woman.
In embracing (and, more specifically, in making love) we are attempting to bring two halves together and thus be more complete and happy. If we find that one person who is our "other half," this is an especially amazing experience, and we find ourselves constantly desiring to be in each other's presence.