Id have to say creative inspiration, things that interest me and the possibility to try new techniques!
I mostly build WW2 armor/figures/dios but have been drawn outside this, by things that looked cool or I took a sudden fancy to!
For a long time I had no interest at all, in Panthers, and didnt find them particuraly cool either, but my last model finished, was a Panther. The Hetzer went in this class as well ... got quite a few plans for that now!
The creativity side, tends to draw me more towards dios and bases. I see things in real life, books,TV, conversations, other models,etc, etc, etc, and I want to try it, or to expand on it, or ....
Sometimes a new technique is best suited to a particular environment or time or subject, so I´ll happily build for that (although I have no particular interest in it), just to try the new technique(s).