We use long molecular dynamics simulations of methane molecules in explicit water at three different temperatures at pressures of 1 and 4000 atm to calculate entropic and enthalpic contributions to the free energy of methane-methane association. In agreement with previous simulation studies, we find that the contact minimum is dominated by entropy whereas the solvent-separated minimum is stabilized by favorable enthalpy of association. Both the entropy and enthalpy at the contact minimum change negligibly with increasing pressure leading to the relative pressure insensitivity of the contact minimum configurations. In contrast, we find that the solvent-separated configurations are increasingly stabilized at higher pressures by enthalpic contributions that prevail over the slightly unfavorable entropic contributions to the free energy. The desolvation barrier is dominated by unfavorable enthalpy of maintaining a dry volume between methanes. However, the increasing height of the desolvation barrier with increasing pressures results from entropy changes at the barrier configurations. Further resolution of the enthalpy of association shows that major contributions to the enthalpy arise from changes in water-water interactions and the mechanical work (PDeltaV) expended in bringing the methanes to a separation of r. A connection of these thermodynamic features with the underlying changes in water structure is made by calculating methane-methane-water oxygen triplet correlation functions.