Why does a neutralization reaction occur?
The driving "force" is the formation of water from the combination of two opposite charge ions: a proton coming from the acid and an hydroxide ion coming from the base.
This combination reduces the energy of (stabilizes) the system (its internal energy and enthalpy derease) of around 58 kJ per mole of water formed if the acid and the base are strong.
This amount of energy is released in the environment as heat during the neutralization, that is an exothermic reaction.
The amount of heat per mole of water (molar heat of neutralization) is less for weak acids or bases.