In my chemistry textbook it is said that noble gases have stable configuration because they have achieved the maximum outermost shell configuration, which corresponds to the $ns^2np^6$ configuration. The question arises is: Why is it so?( Surely I'm not expecting answers like its the way the nature works,etc.) Is there any underlying theory that explains it ?Currently I can think only of the exchange energy associated with such configuration (it is explained quite briefly in the chapter on atomic structure in my textbook) , but I think that there may be something more to it and that's what I'm asking for. (An elaborated answer would help quite a lot) [Note that I tried finding more about it on this Wikipedia page but that doesn't help]