You can turn the tables and say that you are curious about their interest in your reasons.

It's good to have one reply to offer no matter how many times the question is asked, such as "This is what I want to do at this time."
"But why?!", the other asks... same reply: "This is what I want to do at this time."
By repeating the answer, you are also closing the discussion on the subject.

After repeated questions, you can say with a smile that you've said all that you want to say about it.