Physiologically, I think it's something like this:
Low cadence / high gear stresses your lower body as if you were hauling a heavy weight, as each turn of the crank has to move your body + bike farther.
High cadence / low gear stresses your body as if you were jogging or running without a heavy weight, as each turn of the crank doesn't have to move your body + bike as far.
It's a rough analogy, and it breaks down when you look at impact (runners hurt their feet, but cyclists don't; weightlifters hurt their backs, but low-cadence cyclists hurt their knees), but I think it helps.
Me? I'm learning to spin, and I'm glad.