Why is fluorine so reactive?
Because it is an oxidizing agent........
And in fact, elemental fluorine is the most potent oxidizing agent on the Periodic Table. And typically, non-metals are oxidizing agents which ACCEPT electrons, and are thereby reduced. The fact that both oxygen and fluorine occur to the RIGHT of the Periodic Table as we face it, and thus have HIGH effective nuclear charge, rationalizes the tendency of these species to be reduced.
Metals are generally electron-rich materials that undergo oxidation; they are thus REDUCING AGENTS. On the other hand, difluorine and dioxygen routinely undergo reduction to give anions.....
Note that I am guessing your level of study. This is approx. 1st year chemistry, or advanced A-level.