Why not?
1. Lit. Please explain your negative answer. Mother: No, you can't. Mary: Why not? Sue: Could I have another piece of cake? Mary: No. Sue: Why not? Mary: I want it.
2. . Inf. I cannot think of a reason not to, so yes. Bob: You want to go to see a movie next Friday? Jane: Why not? Fred: Do you feel like wandering over to the bowling alley? Tom: Why not?