Whenever I have dealt with that question (and I have dealt with it many times, and even now as an MA student in a history program), I have always tried to look back to the things I was really into in high school and college, and try to understand why I dedicated so much of my time to those pursuits.

For my MA thesis, I decided to research and write about a place that I visited, because the experience of being there was extremely powerful. I'm glad I made that decision, because of the deep personal connection I feel with that place. I would recommend attempting something similar: researching a topic that is powerful that you need to know more about.

Hope this helps.