Nucleophilic substitution in aryl halides is facilitated by electron withdrawing groups while electrophilic substitution is facilitated by electron releasing groups. Why? 

Explanation

Nucleophilic substitution in aryl halides takes place via carbanion as intermediate, which are stabilised by electron withdrawing groups.
Electrophilic substitution takes place through carbocations as intermediates, which are stabilised by electron releasing groups. The rate of the reaction increases if the intermediate involved in this reaction is stabilised.