We clarify the nature of coupling between surface plasmon polaritons (SPP) and transmitted light in metallic nanoslit structures. The coupling strength is found to be the product of the geometric opening ratio, the aperture momentum, and the Fabry-Perot factor. We determine the effective coupling, which includes corrections due to other SPPs, and show that this effective coupling causes enhanced transmission with redshifted or blueshifted transmission peaks. Without coupling, SPP is proven to suppress transmission due to the equipartition of diffraction orders. These results show good agreement with experiment.