Surface self diffusion has been simulated using both Lennard-Jones and Stillinger-Weber potentials in order to explain why experimental activation energies and pre-exponential factors increase greatly at large fractions of the substrate melting point. With both potentials, these effects are observed clearly in simulations and are caused by the proliferation of adatom-vacancy pairs at these temperatures. At the very highest temperatures near melting, the activation energy and prefactor decrease again as the diffusion becomes liquid like.