why carbon always form covalent bond?

The carbon atom needs to either gain or lose 4 electrons to complete its valence shell. But it is difficult for it to lose 4 electrons as it would require about 6200 joules of energy and for gaining 4 electrons it would require more. So it is preferred thermodynamically to form covalent bonds where electrons are shared. But it should also be kept in mind that some hydrocarbons may exist as ionic compounds with metals. For example, CaC_2, which exists as Ca²⁺ (acetylene ion)^2-.

A carbon atom is tetravalent .it has 4valence electrons .it cannot gain electrons as it could lead to low stability as the nuclear charge cannot outweigh the net charge.because of similar reasons it cannot lose electrons so carbon forms compounds by sharing the valence electrons with other elements