Why does hydrogen bonding occur in water?

Hydrogen bonding results from the great difference in electronegativity between oxygen and hydrogen. Electronegativity is the ability of an atom to pull electrons towards itself in a covalent bond. As oxygen is far more electronegative than hydrogen, much of the electron density is situated on the oxygen atom in water, creating a polar molecule. Hydrogen therefore ends up with a partial positive charge, and oxygen with a partial negative charge. Hydrogen is a very small atom, so the charge density is concentrated. This allows hydrogen atoms in water to form weak intermolecular bonds with a lone pair of electron on oxygen. These bonds may be weak, but relative to other intermolecular bonds such as london dispersion forces they are much stronger. They are also very numerous - each molecule in liquid water will have an average of just under 4 hydrogen bonds at any one time.