The theme I'm into at any given time can vary, and is often influenced by movies and documentarires. Currently the theme is Napoleonics, but I've finished watching all the Sharpe and Hornblower DVDs so the theme may soon change.
The fact that Napoleonics are easy to find, usually cheep and have much variations. I've an Italeri French army in Silver, half in blue and now they are coming out in Tan. I often collect by color.
There are several styles of French as well. I find Hat, Streletes and Airfix fit well, while ESCI goes better with Italeri and Zvezda.
Why? Well, mainly for I'm interested in a particular period, and read about it. (my interest being quite large, the fall of Rome till the fall of Berlin, I do collect quite a lot.) Recently we had to do an essay for school on Ernest Hemingway, which got me into buying some Spanish civil war sets. And this weekend I decided to focus my ww2 efforts on Italy for the coming months, just because I like the country.