Why the power of periodic signal is square? I have DSP in my academics and while going through the video lectures i am stuck regarding power of periodic signal.

Its mentioned as 

$$P_{\text{avg}}= \frac{1}{N}\sum_{n=0}^{N-1}|x(n)|^2$$

my doubt is why we have to mention $|x(n)|$ as square ?

Note: I am having format issues please edit my Question and put it in format. thank you.
Power is proportional to the squared amplitude of a signal. This "power" concept comes from thinking of the signal $x[n]$ as representative of a voltage or current waveform. In that model, the power dissipated by applying $x[n]$ to a one-ohm resistor is the value of $|x[n]|^2$ averaged over one period.