Negative Input Resistance of Switching Regulators

Developers of switching regulators and switch mode power supplies attach great importance to the efficiency of their circuits. However, at the end of their development phase, they can encounter unpleasant effects such as unwanted oscillations at the input. These unwanted effects can happen even when the switching regulator produces a constant output voltage under all conditions. So, why does the input of the switching regulator tend to oscillation under certain circumstances?