Well, when I moved to Hungary there were not a lot of non-Hungarians there. And it wasn't a tourist destination for anyone but the Germans and Austrians. So I needed to learn Hungarian just to order a beer at a bar.
The place was also totally corrupt. So I had to know who to bribe and when and how much.
And then there was just the geeky fascination of learning such a weird language. I wanted to know more.
So between the various native girlfriends I had, and a week-long intensive Hungarian course over the winter holidays at a university, and just living every day in the culture, I became fluent in Hungarian.
Trying to learn Serbo-Croat was just a part of working so much in the Balkans. I used to travel for a week or two out of every month throughout the region, driving a truck around to the different military bases. So I would often stop at little roadside diners where nobody spoke English, and have to order food, drinks, get directions, etc. I had to bribe officials, get fuel for the vehicle, deal with border guards, and stuff like that as a part of daily life there, So I picked up what I could from my friends that spoke Serbo-Croat.