Clar's rule of polycyclic aromatic hydrocarbons

Clar’s rule states that the Kekulé resonance structure with the largest number of disjoint aromatic π-sextets, i.e., benzene-like moieties, is the most important for characterization of properties of polycyclic aromatic hydrocarbons (PAHs). What is the reason for the validity of Clar’s rule?

I mean why does polybenzoid structures follow Clar’s rule (what is the mechanism lying behind it) and what is the reason for the stability of these structures?