Here's why:

* Electronegativity: Fluorine has the highest electronegativity of all elements (4.0 on the Pauling scale). Electronegativity is a measure of an atom's ability to attract electrons in a bond. Oxygen has a high electronegativity (3.44), but it's still lower than fluorine.

* Effective Nuclear Charge: Fluorine has a smaller atomic radius than oxygen. This means its electrons are closer to the nucleus, experiencing a stronger pull from the positively charged protons. This stronger attraction is referred to as effective nuclear charge.

In simple terms, fluorine "wants" electrons more strongly than oxygen due to its higher electronegativity and stronger pull from its nucleus. This is why fluorine is the most reactive non-metal element and forms very strong bonds.