A lot of it is serendipity. Mature historians may claim that they are guided by the desire to bolster their position in an larger academic argument, and that may be more true for some than for others, but I suspect chance discovery (or piqued questions as much as facts or insight) is most often the guiding force.

I always told students to study what they find interesting, because interest is a prime motivating force. I also encouraged them to be open to new interests, because that is the best way to broaden one's own horizons. I have found it useful to consider my own unique background: I have a background in several subjects through life experience and/or readings, giving me some expertise in b, l, and q. Since not many people are likely to combine those three things, I can focus on where those three things intersect so that I can make a contribution, lending insight and expertise that may be unique.