High Selectivity of Filter
Let`s start with oscillator circuits. The step response of an oscillator is a continuous sinusoidal wave which can be described by exp(jwt). 

That means: The damping term "sigma" in the complex frequency variable (s=sigma+jw) is sigma=zero. Hence, in the complex s-plane the pole pair is (theoretically) direct on the imag. axis. This is because the solution of the characteristic equation (time domain) is the exprssion exp(st). From system theory we know that the charact. polynominal is identical to the denumerator of the sysytems transfer function T(s). Hence, the poles of T(s) are identical to the roots of the charact. equation.

Now - if we add some damping to the oscillator, the step,response will be a decaying sinusoidal signal - and this is identical to the step response of a high-Q filter (small damping=high pole-Q). This can be verified very easily using the definition of the pole quality factor Qp and its graphical interpretation in the s-plane.  

As a result, all second-order filters have a complex pole pair (negative real part sigma) for qualitiy factors Qp>0.5. All passive RC combination have quality factors of Qp&lt;0.5 (no complex poles, but two real poles on tthe real axis) .