Why are covalent bonds form when an electron is completely lost or gained from an atom

Covalent bonds are the types of Bond which is formed by sharing of an electron between two atoms of same element or two different elements. The bond which is formed by the completely lost or gained of an atom is known as ionic bond.

Explanation:

The most common example of covalent bond is formed from carbon atom. The valence of carbon atom is 4. Therefore, it is very hard to gain or lose an electron from the carbon atom. Therefore, the carbon tends to share the electron instead of completely losing or gaining the electron.

Ionic bond is very common and it can be formed by many elements from the periodic table. Ionic bond is more stable and stronger when compared to the covalent bond in nature.  

the awnser is d i think

b brown mice / white sand dunes