Here's why:

* Electronegativity: Both nitrogen and phosphorus are nonmetals and have relatively high electronegativity values. This means they both have a strong attraction for electrons.

* Sharing Electrons: Instead of one atom completely taking an electron from the other, they share electrons to achieve a stable electron configuration. This sharing of electrons is the hallmark of a covalent bond.

The specific type of covalent bond (single, double, or triple) will depend on the specific molecule formed. For example, in the compound PN (phosphorus nitride), they form a triple bond.