Explain why fluorine is reactive

Fluorine is a halogen, these group 7 elements are reactive because they require just 1 additional electron to achieve a noble configuration. 

Fluorine is the smallest halogen, and so its outer electrons are closer to the charge dense nucleus, making it easier for the nucleus to attract electrons to it.

Fluorine is the most electronegative element in the periodic table, meaning it has a strong tendency to draw electrons towards it (hence it is able to form compounds with most other elements)