Here's why:

* High electronegativity: Fluorine has the highest electronegativity of all elements. This means it has a strong tendency to attract electrons.

* Small atomic size: Fluorine has a very small atomic radius, making its electron cloud highly concentrated. This allows it to interact strongly with other atoms.

* Strong oxidizing power: Due to its high electronegativity, fluorine readily accepts electrons from other atoms, making it a powerful oxidizing agent.

Fluorine is so reactive that it readily reacts with almost all other elements, even noble gases like xenon and krypton. It's a highly corrosive substance and requires careful handling.