I had this brainwave in the shower last night.
In the beginning of this universe, there was nothing but energy. Then the energy became matter. For every bit of matter formed, a corresponding bit of antimatter was formed. When matter and antimatter touch, they annihilate each other, and turn back into energy. For some reason that scientists are unable to explain yet, there was a bit more matter than antimatter formed. That bit more matter is what you and me and everything that we see are made of.
In the beginning of this monetary system, there was nothing. Then that nothing became money. For every bit of money created, a corresponding bit of debt was created. When debt and money touch, they annihilate each other and turn back into nothing. However, because of the way money is created, there is always going to be a bit more debt than money formed. Because all debt carry interest that can’t be cancelled out by the money that was created. It can be cancelled only with new money that is created, but for which there is also corresponding debt and interest.
That is why there is always going to be more debt than money in the system.