Fluorine is the most reactive element because it has the highest electronegativity. Electronegativity is a measure of an atom's ability to attract electrons. The higher the electronegativity, the more strongly an atom attracts electrons.

Fluorine has an electronegativity of 4.0, which is the highest of all the elements. This means that fluorine atoms have a very strong attraction for electrons, and they are very good at pulling electrons away from other atoms. This makes fluorine very reactive, because it is always trying to gain electrons from other atoms.

The reactivity of fluorine is also due to its small size. Fluorine atoms are the smallest of all the atoms, which means that they have a high surface area-to-volume ratio. This means that fluorine atoms have a lot of contact with other atoms, which increases the chances of a reaction occurring.

The combination of fluorine's high electronegativity and small size makes it the most reactive element. Fluorine reacts with almost all other elements, and it often forms very stable compounds.