I attended a workshop in the past where David Epston, one of the co-creators of narrative therapy, told a story about how when he was training in family therapy he would be asked to provide transcripts of hour long sessions and if during those hour long sessions he had asked more than three questions, he had failed his therapeutic task. Our field has a long history of misunderstanding toward the art of inquiry in therapeutic conversations.
Eventually inquiry did become a more accepted means in therapeutic conversations but some questions still remained off-limits, the most predominate being the "why" question. In Michael White's seminal book Maps of Narrative Practice he explains that this may be due to the fact that "why" questions have traditionally been used as a form of moral interrogation which can be diminishing and demeaning of others. For example: Why did you do this? Why would you think such a thing? etc.
However, White believes "why" questions to be quite beneficial in giving voice to and developing intentional understandings about life including, purposes, aspirations, goals, quests, and commitments. White also believed "why" questions can help people develop more positive identity conclusions seperate of how any particular problem may be trying to define their lives. A possible example would be to ask someone "why" they are not OK with the effects of a particular problem in their life, leading to a richer conversation of what values might be sitting underneath those effects.
White is also clear that because it is rare for people to be invited to reflect on their lives in ways that allow them to have a say in what they value or what certain events might be important to them, "why" questions could be unfamiliar and an "I don't know" response can often be expected. White believes in this case it can be helpful to provide an account of how others have responded to "why" questions, for example: "A couple of weeks ago I was meeting with someone and had asked her why she was so dissatisfied with this similar predicament in her life, she said that ____. Would this description fit for you, or something entirely different?"
I too am a proponent of the "why" question and have found them quite helpful in my own practice. It is my experience that justification "why" questions go a long way toward uncovering and discovering people's understandings of what they value, their knowledge of life, their hopes and dreams, and their own lived experience. All of this leading to richer story development, and more positive identity conclusions.