A very wise man once told me this:

"Before you ask a question, check with yourself how each of the answers will affect you. If all answers will affect you in the same way, you don't need to ask the question."

I don't even remember in relation to what he said it, but I remember that it was a yes/no question that I asked. He replied, "What would you do if the answer is yes?", and I said what I would do. He said, "And if the answer is no?"
"The same," I replied. And then he said that sentence, and I withdrew my question. It affected me strongly.

The best example I remember of myself using this sentence is when a friend of mine wanted to get an AIDS test. "Why?" I asked her, and she had no reason. So I asked her what she would do in case it turns out positive or negative. She said she would keep on living her life the same way in both cases. So I told her the sentence, and explained that if the outcome won't affect her, there is no reason to take the test. The sentence didn't change her mind about getting the test, but it made her think about WHY she was getting the test, and what she would do with the results.