This gives the atom an octet which is energetically more stable because it is a full valence shell. The only two atoms satisfied with 2 electrons in its valence shell is H and He. This is because the first set of orbitals contains only 1 s orbital which can contain 2 electrons. The other levels contain at least 1 s orbital ans 3 p orbitals for a stable valence shell of 8 electrons (2 in each orbital)
