Here's why:

* Sulfur has 6 valence electrons. To achieve a stable octet, it needs to gain two more electrons.

* Covalent bonding involves sharing electrons between atoms.

* By forming two covalent bonds, sulfur shares two electrons with other atoms, gaining access to two more electrons and completing its octet.

For example, in hydrogen sulfide (H₂S), sulfur forms two covalent bonds with two hydrogen atoms.