Relationship between valence electrons and electron configuration
Why is it that atoms must have 8 valence electrons in order to be stable?
Aren't electrons most stable when the orbital is completely filled? Helium's electron configuration is 1s^2, Neon's is 1s^2 2s^2 2p^6, etc. All the noble gases, except for helium, have the p orbital filled.
I'm wondering why the p orbital has to be filled in order for the configuration to be stable. Why, for example, aren't the alkali earth metals stable since they all fill the 2s orbital.
Is it because the electronegativity is too weak? If so, why doesn't boron or carbon tend to form give only 1 electron in boron's case or 2 electrons in carbon's case in order to empty the p orbital and be satisfied with a filled 2s orbital? Surely they have enough electronegativity to hold on to the electrons in the 2s orbital.
Of course, empirical data can tell you that the atom likes to have 8 valence electrons, but my idea of the electron configurations doesn't reflect that, so I probably have the wrong idea.
So what forces are responsible for influencing an atom to have 8 valence electrons, and how does the electron configuration agree with this?