We provide new insights into the lithosphere-mantle coupling problem through a joint modeling of lithosphere dynamics and mantle convection and through comparison of model results with the high resolution velocity gradient tensor model along the Earth's plate boundary zones. Using a laterally variable effective viscosity lithosphere model, we compute depth integrated deviatoric stresses associated with both gravitational potential energy (GPE) differences and deeper mantle density buoyancy-driven convection. When deviatoric stresses from horizontal basal tractions, associated with deeper density buoyancy-driven convective circulation of the mantle, are added to those from GPE differences, the fit between the model deviatoric stress field and the deformation indicators improves dramatically in most areas of continental deformation. We find that the stresses induced by the horizontal tractions arising from deep mantle convection contribute approximately 50% of the magnitude of the Earth's deviatoric lithospheric stress field. We also demonstrate that lithosphere-asthenosphere viscosity contrasts and lateral variations within the lithospheric plate boundary zones play an important role in generating the right direction and magnitude of tractions that yield an optimal match between deviatoric stress tensor patterns and the deformation indicators.