Alkenes are rich source of loosely held pi electrons, due to which they show electrophilic addition reaction. Electrophilic addition reactions of alkenes are accompanied by large energy changes so these are energetically favourable than that of electrophilic substitution reactions.
In special conditions alkenes also undergo free radical substitution reactions.
In arenes during electrophilic addition reactions aromatic character of benzene ring is destroyed while during electrophilic substitution reactions it remains intact. Electrophilic substitution reactions of arenes are energetically more favourable than that of electrophilic addition reaction.
That’s why alkenes prefer to undergo electrophilic addition reaction while arenes prefer electrophilic substitution reactions.
