Why not just let fly once in a while?

You mustn't go out now. Why not?

Why not turn the matter over in your mind?

"You mustn't go out now." "Why not?"

"How about playing catch?" "Sure, why not?"

Look! That shop's just opened! Why not take a look?

A nice day, isn't it? Why not go out for a walk?

It's a nice day, isn't it? Why not go out for a walk?

"How about going for a walk?" "Why not? I'd be glad to."

"You had better not wear the red dress." "Why not?"