Because all atoms need 8 electrons in their outer (valence) shell, carbon, having four valence electrons, achieves stability by bonding with 4 other atoms. When doing this, carbon “gains” four more electrons that bring its total valence electrons count up to eight.
For example:
In CH4, carbon bonds with 4 hydrogens and the bonds between the carbon and each hydrogen “add” 4 electrons to carbons outer shell.
