Almost without exception, all physical objects resonate. Or, to put it another way, almost all objects will vibrate naturally at certain frequencies.
Whereas all passive filters have a gain of less than unity, an active resonant circuit can boost frequencies, making the harmonics at those frequencies louder than they were in the input signal. As you can see, the filter still attenuates the frequencies far above the cutoff frequency, but a band around the cutoff is boosted. The low frequencies are slightly attenuated.