Why are steric effects repulsive?
It's all about the Pauli exclusion principle. Assume the available MOs in each of two approaching molecules are fixed, and that each molecule is in a closed shell configuration (all electrons are paired). The electrons in closed shell systems cannot accept additional electrons that share principal, azimuthal and magnetic quantum numbers, since all the spin QMs are taken up. In fact, other than filling higher energy (LUMO) orbitals there is no way to accept additional electrons while retaining the current structure. The solution is to deform the structure of the molecule so as to generate new orbitals that allow formation of novel MOs. The deformation is associated with a potential energy barrier, since it is assumed that the colliding molecules are in low energy electron configurations. Therefore the instantaneous response to the collision is repulsive.