Why do nonmetals form anions?
it is not only non metals form anion as compounds of iron with -1 valancy have been found, carbon also exists in +4, oxygen also exists in +1
generally non metals have high charge/size ratio.
so they tend to attract electrons .
they usually try to fulfil their octate as to reach noble gas configuration
they have two choices to do that
1) loose electrons
2) gain electrons
as non metals have high charge/size ration removing one or two electrons is easy. but removing all of them is like pushing a mountain .
for this u need superhuman strength i.e. very high energy .
same is case with non metals if u want to rip out all electrons u need extremely extremely huge amount of energy which is not feasible thermodynamically.
if they gain electron the attain noble gas configuration easily as they themself tend to attract electrons