I was also trying to figure out how it was that I became a Berlin specialist. While I remember the moment when I decided to become a Germanist, I don't even remember deciding to focus on the Weimar Republic; I really don't remember making Berlin my main focus. It just happened. When I first visited Berlin (four years ago!) I was much more taken with Paris. But somehow Berlin is the one that has taken over my brain. I feel possessive of the city. I felt glad when the Sartorialist declared his love for the city, and got my hackles up when a few commenters expressed a dislike for Berlin.
You know, I still find it all a little odd. (Of course, I also think it's odd that I have an MA. Also, sometimes it strikes me as surprising that I'm married.)