Question

why carbon formed covalent bonds

Answer 1

Because carbon has 4 electrons in its outermost shell (and because each covalent bond requires a donation of 1 electron, per atom, to the bond) carbon has exactly four bonds to make, and is only stable if all 4 of these bonds are used. This same rule applies to all of the atoms in the second row of the periodic table.

Answer 2

carbons inability to form ionic bonds forces it to form covalent bonds, because carbon has tetra valency i.e. four valence electrons and it's difficult to for it lose them and become an anion , it's also difficult for it to gain and accomodate 4 electrons and become a cation thus it overcomes the problem by Sharing the electrons