why do you need to be in order to be a runner and survive the maze?why maze so terrible,according to newt?

Newt stops tells Thomas that a Runner has to be smarter, stronger, and faster than everyone else. A Runner needs to be bold but not reckless, decisive, and not easily frightened.

The maze is terrible because it changes every night. The stress is what is most frightening. There is always the possibility that a Runner will get lost and not be able to make it back.