Why does Fido shake?

The Wet Dog Shake: Scientists (apparently don't) Uncover Secret Formula - ABC News: "
An important physical factor not mentioned in the article: wet fur physically is "loosely coupled" to the interior of the dog. The "wet fur shake" is similar to a flywheel driven by a spring, and made to oscillate rotationally. The rotational intertia of the fur and the nature of its coupling to the dog's musculature result in a resonant system which oscillates easily at one frequency and with greater difficulty at different frequencies. Increasing the radius of the flywheel tends to increase its rotational intertia and reduce the resonant frequency of the system.

Why does Fido shake himself dry at a particular rotational frequency? Because he can get enough rotational velocity to disperse the water only if he shakes near the system's resonant frequency. In other words: why does he shake at that frequency? Because he can. Why doesn't he shake at a different frequency? Because it's hard, and it doesn't do anything for him. Good dog.