We use why as a short response when someone says something that surprises us or when we want to know the reason. Generally, why is used in response to a positive statement:
  • Jim: I have to miss the party tomorrow.
  • Tom: Why?
  • Jim: Because I have to work.
  • Kim: Today’s meeting was cancelled.
  • Bill: Why?
  • Kim: Because the CEO has some urgent matter to attend to.
In response to a negative statement, we use why not when someone says something that surprises us or when we want to know the reason.
  • Jim: I can’t go to the party tomorrow.
  • Tom: Why not?
  • Jim: Because I have to work.
  • Kim: I heard we won’t have a meeting today.
  • Bill: Why not?
  • Kim: Because the CEO has some urgent matter to attend to.
We also use why not when we want to agree with something that someone said. We think that their suggestion or idea is good, so we can reply with why not.
  • Anne: Let’s order another bottle of wine.
  • Lisa: Sure, why not?
  • Serena: This café looks nice. Let’s go in.
  • Jenny: Ok, why not?