Here's why:

* Sulfur's electron configuration: Sulfur has 16 electrons, with the configuration [Ne] 3s² 3p⁴.

* Noble gas configuration: The nearest noble gas to sulfur is Argon (Ar), which has 18 electrons with the configuration [Ne] 3s² 3p⁶.

* Gaining electrons: To achieve the stable, full octet of electrons like Argon, sulfur needs to gain two more electrons in its 3p orbital.

Therefore, sulfur gains two electrons to become a sulfide ion (S²⁻) and achieve a noble gas configuration.