What keeps an electric current going in a circuit?

I understand that that the two ends of the conducting wire being connected to the positive and negative sides of a load cause the electrons of the wire to be rushed from one end of the wire to the other, but what sustains the current? Why don't all the electrons stop once they meet the positive side and collect there? In other words, why does the current continue after all the electrons have made it to the positive end of the load for the first time?