Why are double bonds more reactive than single bonds?
Double bonds have, in addition to the sigma orbital overlap of a single bond a pair of pi lobes on each of the two adjacent carbons. Any electrophilic or radical atom that comes near a pi lobe will try to share the claim to the electrons the original pi-bond has and will tend to weaken the double bonding in favor of a new single bond. That is how polymerization takes place, for example.
