Why Do We Treat Input and Output Impedances Differently in FET Amplifiers?

[MHR-Love]

Hi guys,
can anyone tell me the reason why we set Vi=0 (short circuit the input side) when we want to find the output impedance Zo but we don't do the same by setting Vo=0 when we find the input impedance Zi?
and why do we let Io=0 (open circuit the output side) to find the gain Av?

thank you

 

[viscousflow]

Think of what is happening in the FET from the point of view of impedance. The collector-emitter side can be viewed as a current source. What are the implications of this?