The essential part is to start treating the grounding as a part of the circuit itself: Each time a part of your circuit makes a connection to common it will not have a reference of ideal zero volts. Instead, this connection is just the other end of conductor connected to the only real common point that can exist in the system - a conductor with resistance. Since the circuit stage's current goes through this resistor, then according to U=IR the "common reference" point of the stage must have a voltage potential. It would be better to refer to these points as "common returns" not as commons.