The world was inhabited by people. Not the kind we're used to though. There were men, women, and people who were both male and female. Not so strange? These people weren't considered "normal" by any means; they were actually two people sharing the same body. So each "person" had two heads, four arms, and four legs, and these people were powerful. So powerful in fact that Zeus and the other gods feared their strength. And soon their fears became a reality.
Trouble arose when the gods heard that these beings they created were thinking about climbing to heaven to replace the gods themselves. Of course the gods were furious and some of them said the easiest thing to do would be destroy humankind. But, Zeus patient , and wise listened to their gripes and groans and plans and came up with his own. He proposed that they should cut all these beings in half, this would benefit the gods and in many ways. First, it would immediately double the number of people making offerings and secondly, it would weaken the race, effectivly cutting their strength in half.
All the gods loved the idea and the humans were divided in two. The human race was confused, upset, and feeling empty. Zeus was filled with compassion and made a decison to help ease their pain and suffering. He made it so that each half was able to have sex with their opposite and in their brief moments of love making they would be able to return to eachother, becoming lostin their symbolic oneness if only for a short while.
And since that day the human race has been in search for their soulmate.