Plato wrote in his Symposium that humans have been looking for their soul mate ever since Zeus cut them in half. In his mythic story, Plato describes a world where there were men, women and people who were both men and women. Apparently, humans began discussing how they could climb up to heaven and replace the gods. The gods were upset by this and discussed what should be done. The simplest solution would be to destroy mankind, but Zeus came up with a better idea. He suggested cutting all human beings in half. This would serve two purposes. First, it would immediately double the number of people making offerings to the gods. Second, it would weaken the humans, so they would not be able to carry out their plan. Zeus' idea was accepted, and the humans were all divided into two.
...and so, the humans wandered the earth evermore, searching for their other half.