"Why not?" is a colloquial stand-alone answer in English. It can expresses
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?)
A: Why are you so fancy today?
B: Why not?
(as opposed to "B: Because I have a date tonight." for example)
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?