Why is neutralization reaction exothermic in nature? [duplicate]

Why is heat liberated in such reaction. Isn't it just a simple rearrangement of ions? Hydrogen ion joining hydroxyl ion.

You are right in that the reaction is $\mathrm{H}^+ + \mathrm{OH}^- \rightarrow \mathrm{H_2O}$, but recognise that bonds are formed in this reaction. Bond formation releases energy (and similarly bond breakage is endothermic), and thus neutralisation reactions are exothermic.