How are bonding orbitals formed?
A bonding orbital is formed when two atoms approach each other and the overlap between atomic orbitals results in constructive interference.
The constructive interference causes the electron density between the two atoms to increase. This lowers the kinetic and potential energy of 1 or 2 electrons in the resulting bonding orbital. Because the energy of the system is lower when the atoms are near each other than when they are pulled apart, we say that a bond is formed.