I went to a job interview about ten years ago and I got this question. I had carefully filled out the application and carefully considered the situation. I believe I had more self-awareness than many thirty-four year olds, but this was still a tough question for me. I had not read Interviewing For Dummies.
I'm still wondering about this question and my answer to it. I'm not sure if that question was in vogue ten years ago or what.
I think that there are two ways to discern your passion. One is to ask, "what do you care about?" What gets you going or excites you? The second, is to look back at your life. What have you invested your time in?
Yes, I got the position, that was a job I had dreamed of having years earlier. It turned out though that from the new vantage point or vista, I could see much farther and my dream got much bigger. From then till now, my passion has been realigned and reformed though.
Here is a quick "what's your passion?" exercise, that I found, if you are interested: