Benzene is stable due to its delocalised ring of electrons - which forms as a result of the carbon-carbon bonds being neither single nor double bonds, instead an intermediate length with electrons in the p-orbitals sticking out above and below the carbon ring. This means that, when compared to cyclohexa-1,3,5-triene, benzene has a much less exothermic enthalpy of hydrogenation - -208kJmol-1 instead of the predicted -360kJmol-1. This means it's a more stable compound, as more energy is needed to break benzene's bonds than cyclohexa-1,3,5-triene.