Here's why:

* Electropositivity: Francium has the highest electronegativity of all metals, meaning it readily loses electrons to form positive ions.

* Location on the Periodic Table: Francium sits at the bottom of Group 1 (alkali metals) on the periodic table. Elements within a group become more reactive as you move down.

* Radioactivity: Francium is radioactive, meaning its atoms are unstable and readily decay. This instability contributes to its high reactivity.

Important Note: Francium is extremely rare and highly radioactive, making it difficult to study and handle.