At any given time your body will have a natural cadence that it will want to go if you are not thinking about it. But it is trainable, you can do high-cadence drills to get your body used to higher cadences. Also, cadence is somewhat speed related. The slower I go, the lowe my cadence will be naturally. It just doesn't feel right to be pushing 100 rpm at 16 mph, but say at 30 it is likely to be over 100. This is because slower speeds require less power (all else being equal) and therefore a the same cadence would result in less torque, i.e. "resistance" on the pedals.
My thought on it also.