* Dissolving and Ionization: When NaOH is added to water, it dissolves and dissociates into its ions: Na+ and OH-. This process involves the formation of new bonds between the ions and water molecules.

* Energy Release: The formation of these new bonds releases energy, which is why the solution gets hot. This energy release is greater than the energy required to break the bonds in the original NaOH and water molecules.

In summary: The reaction is exothermic because the energy released during bond formation in the dissolution process is greater than the energy required to break existing bonds.