Here's why:

* Electronic Configuration: Both alkali metals and hydrogen have only one electron in their outermost shell. This single valence electron is responsible for their similar reactivity.

* Tendency to Lose Electrons: Alkali metals readily lose their single valence electron to form a +1 ion. Similarly, hydrogen can lose its single electron to form a proton (H+), though this is less common than gaining an electron to form a hydride ion (H-).

* Reactivity: Both alkali metals and hydrogen are highly reactive. Alkali metals react vigorously with water and react readily with nonmetals. Hydrogen also reacts with many elements, especially at elevated temperatures.

However, it's important to note that:

* Hydrogen is a nonmetal: While it shares some chemical similarities with alkali metals, hydrogen is classified as a nonmetal due to its physical properties like its diatomic gaseous state at room temperature.

* Hydrogen is a much smaller atom: This difference in size can lead to different chemical behaviors.

Therefore, while hydrogen shares some similarities with alkali metals, it's not considered a true member of the group.