A generalized comeback when someone suggests for you to do something. Especially when said something is:
b) your job but you'd rather it not be
Most effectively used when your friend is right but you don't want to give them the gratification of admitting as much.
Friend1: "I really like this girl but I don't know what to do."
Friend2: "Why don't you just ask her out?"
Friend1: "Why don't you?"