A coupled harmonic oscillator picture is developed for the nonlinear optical response of donor-acceptor substituted conjugated polyenes using the reduced single electron density matrix calculated within the time-dependent Hartree-Fock (TDHF) framework. The mechanism of the optical response is related to a few dominant oscillators which represent collective electron-hole excitations. The nature of these oscillators is analyzed. The optimization of the off-resonant nonlinear susceptibilities beta(0) and gamma(0) and the roles of Coulomb and exchange interactions are investigated.