Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty?

Answer

Due to the rpresence of `6pi` electrons, benzene behaves as a rich source of electroncs thus being easily attached by reagents deficient in electrons.

Explanation

Benzene is a planar molecule having delocalized electrons above and below the plane of ring. Hence, it is electron-rich. As a result, it is highly attractive to electron deficient species i.e., electrophiles.
Therefore, it undergoes electrophilic substitution reactions very easily. Nucleophiles are electron-rich. Hence, they are repelled by benzene. Hence, benzene undergoes nucleophilic substitutions with difficulty.