Here's why:

* Low ionization energy: They readily lose their outermost electron to form a +1 cation, making them strong reducing agents.

* Large atomic size: Their outer electron is further away from the nucleus and weakly attracted, making it easier to remove.

* Weak metallic bonding: The weak bonding contributes to their softness and low melting and boiling points.

This high reactivity means they react vigorously with water, oxygen, and other nonmetals. For example, lithium, sodium, and potassium react explosively with water.

Important Note: Due to their high reactivity, Group 1 metals are typically stored under oil to prevent reaction with air and moisture.