For my part, because all the smart kids in my school did two languages, instead of one language and Art. As you can see from Gallery of Awesomery, not doing Art has paid off.
In the eighties, the two main languages being taught in Australia were still French and German, which was a cultural inheritance from Britain. So I learnt French and German. The year after my year, French and Japanese were also offered together. I would have welcomed the challenge of Japanese in my twenties, but not, I think, in my teens.
That’s why I started. Why I continued with it was falling in love with German culture, working in a discipline that depended on German scholarship, and making excellent German-speaking friends. Which I have continued to do here.