Why does the double bond in alkenes make them more reactive than alkanes, which are single-bonded?

The double bond makes alkenes more reactive than alkanes because the π bond is weak.

A C=C double bond consists of one σ and one π bond.

In a σ bond, the electrons are half-way between the two nuclei. They are in a small volume of space. Their attractions to the nuclei are strong. The σ bond is strong.

In a π bond, the electrons are “off to the side”. They are further away from the two nuclei, so the nuclear attractions are not as strong. A π bond is weaker than a σ bond.

Furthermore, the electrons are “delocalized” over the two atoms. The electron cloud is larger and more diffuse, and the σ electrons are almost buried inside the π cloud.

All these factors make it easier for an electrophilic (electron loving) reagent to attack the π electrons.