Here's why:

* Covalent bonding involves the sharing of electrons between atoms. Nitrogen and chlorine are both nonmetals, and nonmetals tend to form covalent bonds.

* Nitrogen has 5 valence electrons and needs 3 more to complete its octet.

* Chlorine has 7 valence electrons and needs 1 more to complete its octet.

* To achieve stability, nitrogen shares one electron with each of the three chlorine atoms, and each chlorine atom shares one electron with nitrogen. This sharing of electrons forms three covalent bonds between the nitrogen and chlorine atoms.