Theoretical studies of hydrogen-bonding based on cluster models tend to overlook the peripheral monomers which are influential. By revisiting thirteen hydrogen-bonded complexes of H2O, HF and NH3, the "push-pull" effect is identified as a general mechanism that strengthens a hydrogen bond. Enhanced Lp(X) -> sigma*(X'-II) charge transfer is proved to be the core of the "push-pull" effect. The charge transfer can convert an electrostatic hydrogen bond into a covalent hydrogen bond. (C) 2017 Elsevier B.V. All rights reserved.