From what I can remember I had to if I wanted to communicate with the humans around me, and I did, so I learned.
Translating lyrics from songs I wanted to understand, reading books once in each language, tv shows with subtitles on, student work exchange and extensive stay immersed in given language/culture.
Mix of everything really.. courses in the beginning, apps for vocab, and at some point I started reading newspapers every day even when I understood maybe half of it.