It's just whatever tickles your fancy. I started out loving WWI in some strange sense of burgeoning French nationalism out of High School which gradually expanded into a knowledge of the British and Germans alike on the Western Front. As I became more knowledgeable about the war I became interested in the leadup to it and the context behind it, the Franco-Prussian War and the Napoleonic Wars and the Concert of Europe. Over time it just naturally branched out from there until here I am today with an above average knowledge of post-French Revolutionary Europe up through WWI and the military theory stretching from a couple decades prior to the Revolution until about WWI.

You'll notice at no one time did I sit down and scratch my chin or throw a dart at a board or spin a wheel of fortune. I found one topic, it lead to more knowledge in that topic, I began exploring other areas of interest related to the topic and those branched out naturally. What started out as me being basically (entirely) focused on WWI French military doctrine has now lead me to now where I'm almost entirely focused on 18th century France and Prussia pre-Revolution as my tastes begin to expand even further beyond what this flair says.

I know it's cheesy but it really is an ever expanding web of knowledge. I always try to be reading something relevant to my flair (got 2 WWI books and 2 Napoleonic War books and one social history book about German unification I'm going through steadily) because those topics were my first love and I love to stay up to date on them but I'm always trying to become more knowledgeable about something. I read a book and the author has a particularly great section on a certain topic and I get enamored with that and I order a book on that specific topic and so it tumbles on from there.

This is why I get a bit frustrated at questions to the effect of "where do I start?" or "I don't know where to even begin" or "I want to study history, what do?" and the like; there is no answer to it. You just choose something that piques your interest and dive in. You read an introductory book or two on said topic and you'll naturally know where to go from there when that little tingle in your brain says "woah that's fucking cool." It took me over a year and a half now of cranking through 1-3 books a month before I found a bed of history I'm remotely comfortable in (1789-1918) so it does take time and a lot of personal exploration.