Here's why:

* Noble Gases: Helium belongs to a group of elements called noble gases. These gases are located in Group 18 of the periodic table.

* Full Outer Shells: Noble gases have a full outer shell of electrons, making them very stable and unreactive. They don't readily gain or lose electrons to form chemical bonds.

While other noble gases like neon, argon, krypton, xenon, and radon are also non-reactive, helium is considered the least reactive of them all.