Why does current have to return to its source in a circuit?
If there is no "return" galvanic connection between two independent circuits, those circuits will rapidly charge to the same voltage and then current will stop flowing. The speed at which they charge is down to their relative capacitance to ground and their initial potential difference. Imagine two buckets of water (connected via a pipe at their bases and a tap that is turned off preventing flow). The two buckets have different heights of water but, after the tap is opened, the levels equalize and no more water flows. Water-height = voltage, water-flow = current.

If there is a galvanic link i.e. the two circuits are electrically connected (even by high value resistors), current will continue to flow because there will always be a potential difference generated by the current flowing through the resistors maintaining a static voltage difference. 

If they are galvanically linked and have the same potential, current will not flow.