Here's why:

* Ionization Energy: Lithium has a lower ionization energy than beryllium. This means it takes less energy to remove an electron from a lithium atom, making it easier to form a positive ion and participate in chemical reactions.

* Electropositivity: Lithium is more electropositive than beryllium. Electropositivity refers to the tendency of an element to lose electrons and form positive ions.

* Atomic Size: Lithium atoms are smaller than beryllium atoms. This smaller size leads to a stronger attraction between the nucleus and the valence electron, making it easier for lithium to lose its electron.

In general, reactivity increases down a group and decreases across a period in the periodic table. Lithium is in Group 1 (alkali metals) and beryllium is in Group 2 (alkaline earth metals). Lithium is higher up in the periodic table than beryllium, so it is more reactive.