Ooh, this is probably related to how a child learns languages very well if they are taught in the environment where only that language is spoken. I guess as an adult it also helps a lot? For me, it sorta worked, because after I watched a lot of Chinese dramas I got better at Chinese, and I got a starting vocabulary (and interest) in learning Japanese. I apparently speak quite well, according to some of my native speaker friends, but I'm still quite hopeless at writing, so I guess it doesn't help in that department!