Why do molecules with multiple bonds tend to be more reactive?

Molecules with multiple bonds are often more reactive because they contain pi ($\pi$) bonds, which create regions of high electron density outside the direct atom-to-atom bond axis.12

In a double bond or triple bond, these $\pi$ electrons are located farther from the positively charged nuclei than the electrons in a sigma ($\sigma$) bond, making them more weakly held and more accessible. This increased electron density attracts electron-deficient atoms or groups, which initiates the process of bond-breaking—a necessary prerequisite for a chemical reaction.341

While the overall multiple bond is strong, the $\pi$ component is susceptible to rupture, allowing the molecule to undergo addition reactions where the $\pi$ bond is broken to form new, stronger $\sigma$ bonds. This reactivity is a defining feature of functional groups in organic chemistry, distinguishing these unsaturated molecules from more stable, saturated compounds that contain only single $\sigma$ bonds.41