In addition to some people naturally having more difficulty learning on their own, there are certain subjects that pretty much REQUIRE interaction with other people in an academic context for you to be able to grow and use your brain in more complex ways, at least after a certain level. Music, history, anthropology, psychology, literature, film studies, writing, and other more humanities-geared subjects are geared more towards discussion and critique of each others' ideas. And even in maths and sciences, performing studies and discussing the results of yours and others' studies or creating and discussing theories becomes the main focus after a while.