  We discuss various microscopic mechanisms for level attraction in a
hybridized magnon-photon system of a ferromagnet in a microwave cavity. The
discussion is based upon the electromagnetic theory of continuous media where
the effects of the internal magnetization dynamics of the ferromagnet are
described using dynamical response functions. This approach is in agreement
with quantized multi-oscillator models of coupled photon-magnon dynamics. We
demonstrate that to provide the attractive interaction between the modes, the
effective response functions should be diamagnetic. Magneto-optical coupling is
found to be one mechanism for the effective diamagnetic response, which is
proportional to photon number. A dual mechanism based on the Aharonov-Casher
effect is also highlighted, which is instead dependent on magnon number.
