Here's why:

* High Electronegativity: Oxygen has a high electronegativity, meaning it strongly attracts electrons. This makes it very reactive, readily forming bonds with other elements.

* Abundance: Oxygen makes up roughly 21% of Earth's atmosphere, meaning there's a lot of it around to react with.

* Oxidation: Oxygen is the primary reactant in oxidation reactions, which are fundamental to many processes on Earth, including respiration, combustion, and corrosion.

While other reactive gases like ozone (O3) and chlorine (Cl2) are present in the atmosphere, their concentrations are significantly lower than oxygen's, making oxygen the most reactive gas overall.