Why do covalent bonds form when atoms share electrons?

Covalent bonds form because the resulting system achieves a lower, more stable energy state than the atoms would have if they remained widely separated.12

The Energetic Basis of Bonding

In the simplest view, a covalent bond occurs when two atoms share a pair of electrons that reside between their nuclei. Because these electrons are attracted to two positive nuclei simultaneously rather than just one, the system reaches a lower energy level.12

More comprehensive quantum mechanical models, such as molecular-orbital theory, explain that when atoms approach, their atomic orbitals combine to form new molecular orbitals. The electrons occupy a "bonding molecular orbital," which is at a lower energy level than the original atomic orbitals. Because the formation of this bond releases energy, the process is energetically favored.1

Achieving Stability

From the perspective of the octet rule, atoms often share electrons to complete their outer electron shells, mimicking the stable configuration of noble gases. While this rule is a useful guide, the fundamental driver of covalent bonding is the overall reduction in the system's total energy.23