I’m not sure if you learned this but if you look at the periodic trends. I believe it has the greatest . If you were to combine it with any other element, it’s electronegativty difference (also called ionic character) is the greatest and therefore it is the most reactive.
Another trend that would show why it is the most reactive is the periodic trend of electron affinity. I believe fluorine has the greatest electron affinity and therefore is the most reactive.
In basic chemistry, fluorine is really reactive because it only requires 1 electron to obtain a full valence shell. The akali metals are really reactive also but you said the most reactive non-metal.