Why benzene survives most nucleophilic attacks
Benzene is unusually stable because its six pi electrons are delocalized over the ring in a low-energy, aromatic configuration. Nucleophiles ordinarily attack electrophilic carbons by adding to the pi bond. Benzene resists this because it would break the aromaticity and create an unstable, non-aromatic intermediate. Instead, benzene reacts with electrophiles (positive species) in a process called electrophilic aromatic substitution.
The mechanism: sigma complex and re-aromatization
An electrophile (such as Br+, NO2+, or R+ from a carbocation) attacks a pi bond of benzene, forming a sigma complex (also called an arenium ion or Wheland intermediate). This cyclohexadienyl cation has a positive charge delocalized over three carbons and is highly stabilized by resonance. A base then removes a proton from the same carbon that was attacked, re-forming the aromaticity. If the proton is removed from the carbon next to the electrophile, the original substitution occurs (most common). If removed from a different position, rearrangement products form, but these are usually minor.
Activating and deactivating groups direct the substitution
Substituents already on the benzene ring control which position is attacked next. Electron-donating groups (alkyl, alkoxy, amino) activate the ring and direct incoming electrophiles to ortho and para positions because those intermediates are stabilized by resonance from the activating group. Electron-withdrawing groups (nitro, cyano, carboxylic acid) deactivate the ring and direct incoming electrophiles to meta positions because ortho and para intermediates are destabilized by the electron-withdrawing group. This selectivity is exploited in synthesis to build complex aromatic molecules step by step.