What drives an atom to form a covalent bond? nuclear charge or a filled outer shell?

The tendency as for why atoms will want to form a chemicals bond by sharing certain number of valence electrons is in order for the atoms involved in the chemical bond to achieve a filled outer shell. To have an occupied electron shell, in most cases a stable octet, so that the atoms will become more stable and not as reactive.

Relationships are difficult because people choose to walk away instead of having to try harder.