Cyrus

When two atoms form a bond with each other and if this bond is formed by the contribution of one electron from each atom, such a type of bond is called a covalent bond. Talking about carbon, it forms four covalent bonds, e.g. CH4. Carbon has 4 electrons in its valence shell. Thus, to attain stability, it must either gain 4 electrons completing its octet and stable electronic configuration or lose 4 electrons, resulting in a stable electronic configuration.
But when it gains 4 electrons, excess negative charge is generated, and there will be a lot of repulsions between the electrons, and thus it won't be stable. If it loses 4 electrons, an excess positive charge is generated, which is also not stable.
Hence to attain stability, it can't gain or lose electrons, making a bond by sharing electrons (i.e., covalent bond) to complete its octet, leading to stability.