Benzene contains six pi electrons which are delocalized in six p orbitals above and below the plane of the benzene ring.
The six pi electrons obey Huckel's rule so benzene is especially stable. This means that the aromatic ring wants to be retained during reactions. Because of this benzene does not undergo addition like other unsaturated hydrocarbons. The non-aromatic product would lose the extra stability that is present in aromatic molecules.
However, benzene can undergo electrophilic aromatic substitution reactions because aromaticity is maintained in the product.