Here's why:

* Benzene is a highly stable molecule. The delocalized electrons in the benzene ring create a resonance structure that contributes to its stability.

* The enthalpy of formation is the heat change when one mole of a compound is formed from its elements in their standard states. In the case of benzene, the standard states are solid carbon (graphite) and gaseous hydrogen.

* The formation of benzene releases energy, indicating an exothermic process. This energy release is due to the formation of strong C-C and C-H bonds in the benzene molecule, which more than compensates for the energy needed to break the bonds in the reactants (graphite and hydrogen).

Therefore, the enthalpy of formation of benzene is negative, indicating an exothermic reaction.

It's important to note that while the enthalpy of formation of benzene is exothermic, the combustion of benzene is highly exothermic. This is because the combustion process involves breaking strong C-C and C-H bonds and forming weaker bonds with oxygen, releasing a significant amount of energy.