Studying with a Study Partner

Why does it seem to be the norm to study Torah with a study partner instead of just sitting alone with a book?

People study together with a partner in the classic chavruta (literally, comrade) style, discussing and analyzing ways to understand complex subject matter. Explaining an idea to another person forces one to clarify it in his own mind. The debate between the study partners will refine it even further. As one of the greatest Talmudic scholars of the last century, Rabbi Chaim Soloveitchik, said, "The degree to which one is able to explain something is directly proportional to the degree to which one understands it." In addition, studying with a partner provides an opportunity to see ideas from someone else's perspective.