I agree that "why?" questions tend to put me on the defensive.

I have also had the experience that a "why?" question didn't lead to fruitful insight, but a differently phrased question did. In one of my sessions, I had told my T, "Honestly, I don't want to be in a world where there's no perfect love. It's not worth it." "Why not?" my T said. I rarely say "I don't know" in session, but I thought and thought and just couldn't come up with any reason. So I said it. And she said, "Let me ask a better question. What would a world without a perfect love be like?" And then I was off and running with answer after answer, that gave insight into where this thought was really springing from inside me.