Coordinate covalent bonds are bonds on which both electrons from one atom.
But why does this happen? Some may think it is because one of the bonding atoms have strong electronegativity. But experimental evidence suggests otherwise.
For example, carbon monoxide. The EN of carbon is 2.6, and oxygen 3.4. This may lead you to think that carbon will donate a pair of electron, but on Princeton.edu:
Carbon monoxide (CO) can be viewed as containing one coordinate bond and two "normal" covalent bonds between the carbon atom and the oxygen atom. This highly unusual description illustrates the flexibility of this bonding description. Thus in CO, carbon is the electron acceptor and oxygen is the electron donor.
Nevertheless, in some cases like NO2, it does follows that the less electronegative atom domates electron. But why??