Why is diene more reactive than alkene?
It is not only the diene itself which is stabilised by resonance, but on electrophilic addition the carbocation formed is also stabilised by resonance if the diene is conjugated. Thus, forming the intermediate for a conjugated diene is easier than forming the intermediate for the alkene, even though the starting compound itself is more stable in case of conjugated dienes and therefore conjugated dienes react faster.