Question

why octet or duplet is the stable condition in atom's outtermost shell

Answer 1

octet or duplet is the stable condition in atom's outermost shell because

once a atom reaches it octet or duplet its valency is filled there will be no electrons to lose nor to gain because of which the atom does not try to react with other atoms

every atom tries to attain stability by obtaining octet or duplet they either lose or gain electrons if it is not attained the atom becomes highly unstable in order to reach that state

1)reactivity depends on the valency of the atoms
2)Valency is achieved
3)The atom can't participate in reaction

but some of the atoms does not follow this which is the drawback for the theory of octet some of the atoms follow expanded octet which means they will have more than 8 electrons in a central atom 

Answer 2

's'orbital has chance only to fill 2 electrons. So, it is called duplex configuration. So, it has no more chance to fill more than 2 electrons.So,it is stable.
's'orbital has chance only to fill 2 electrons and 'p'orbital has chance to fill only 6 electrons. Total is 8 electrons. So, it is card octet configuration. It has no more chance to fill more than 8 electrons. So, it is stable.