How to say "Why not?" in Korean?

Question

"Why not?" is a colloquial stand-alone answer in English. It can expresses

1. An answer to a "why" question, explaining that there is no specific reason. For example,

   A: Why are you so nice with me today ?

   B: Why not? (Do I need a reason to be nice?)

   or

   A: Why are you so fancy today?

   B: Why not?

   (as opposed to "B: Because I have a date tonight." for example)

2. An agreement to a proposition. Example:

   A: Shall we see a movie together?

   B: Sure. Why not? (Could there be a reason I don't want to see a movie with you?)

Are there similar colloquial expressions in Korean that are commonly used to express these two meanings?

As a follow-up questions, in the situation 1., "why not?" can be used to show some annoyance from the question, as well as a genuine absence of reason (depending on the tone of the answer). Is there a way to express such nuance in Korean?

Answer 1

Well I don't think there would be an exact match; the closest could be "그렇지 않을/못할 이유라도 (있나)?" This might be used fairly well in situation 1, but in situation 2 it wouldn't be used often. B would end his talk with "sure".

Answer 2

For Case 1, I believe "그럼 안돼(요)?" would work. Literally, "Is it not allowed?"

Just as you mentioned at the end of the question, this phrase can also indicate annoyance; it could also mean a genuine question asking whether something is allowed, so use with care. (In particular, I think the phrase is more informal than "why not," so you wouldn't want to say "그럼 안돼요?" to mean "why not" in a business meeting.)