Lithium is so reactive because it is what is called a "group 1" element on the periodic table. All the group 1 elements are insanely reactive. They have a free electron and aren't afraid to use it. Rb, Cs and Fr are increasingly rare as you move down the group, but are similarly reactive. These three are often nasty fallout products from nuclear detonations or meltdowns. CS is a particularly bad one.

Try throwing pure Sodium or Potassium in water, and you'll get the same or even more energetic results as Lithium. Most of these elements are bound into compounds, so they don't just randomly catch on fire. They are usually stored in an oil, so they can't react with air.

In high skool, I got in a little trouble when I got some Potassium from our chem lab and flushed in down a toilet at our arch-rival. The principle said he knew it was me because I was the only sophmore smart enough to pull the caper.

This most likely had something to do with why all the lithium batteries were exploding and catching fire, which was very concerning when it happened on airplanes.