Here's why:

* Sulfur's electron configuration: Sulfur has 16 electrons, with the configuration: 2, 8, 6. This means it has 6 electrons in its outermost shell (valence electrons).

* Octet Rule: Atoms tend to gain or lose electrons to achieve a full outer shell of 8 electrons (octet rule).

* Sulfur's preference: To achieve an octet, sulfur prefers to *gain* 2 electrons, which gives it a charge of -2. This forms the sulfide ion (S²⁻).

Sulfur can also form other ions, like S⁴⁺, but -2 is the most stable and common form.