Jisho

not necessarily. e.g. それほどできれば私の手助けの必要があるでも？ --> if u can do That, do you still need my help?
in japanese, it is true that using negation and passive voice is prefered because it is seen as more subtle, polite, elegant, etc. that's why we can have impression that some grammar structure only used with negative form but most of the time, there's no rules about that.