Why is benzene more stable than expected?

All the carbons in benzene are sp² hybridised. The carbons are each bonded to one hydrogen and all bonds to other carbon atoms in the ring are the same length. This bond is in between the bond length of a C=C and C-C bond. This is because there is a system of p orbitals that can overlap to form a delocalised system of electrons above and below the plane of carbon atoms. This system leads to the benzene ring being more stable than expected.