Aromaticity

Huckel's Rule: Planar, fully conjugated, monocyclic systems with (4n+2) p electrons have a closed shell of electrons in bonding orbitals and are exceptionally stable. These systems are said to be aromatic.

Aromaticity adds a lot of stability to a compound, so if it is achievable it is usually attained. Generally this occurs in cases where an aromatic system has lost aromaticity, and in a later stage of reaction regains it by removal of a proton or other means. One common instance where this occurs is in Electrophilic Aromatic Substitution.