A covalent bond is most likely to be formed between two elements which

Atoms with comparable and dissimilar properties may form a covalent bond.
A covalent bond can be created between two atoms that have the same electronegativity or one that is comparable.

For instance, in a hydrogen molecule, a covalent bond is created when two H atoms unite.
In addition, methane forms four C-H covalent bonds from the combination of one C atom and four H atoms.