We can look upon this aspect in two different ways.
1. About individual atom if we consider the role of single atom in bonding , Maximum electron gain or loss is 3 electrons, so if an atom has more than 3 electrons it can not donate or gain electrons thus it follow covalent path by sharing electrons.
2. Intraction between two atoms:- Covalent bond is basically formed by the atoms that have electronegativity difference less than 2 . In this kind of bonding the electrons are shared between atoms, the transfer of electrons do not occurs here.
The Unconditional Guru 