It's not really a great deal. As I understand it there is a high pressure to marry early with people over 25 being considered undesirable. Many of these friends are made in high school. I also found a news source on this topic: http://en.rocketnews24.com/2016/11/30/more-japanese-are-marrying-friends-and-acquaintances-because-they-dont-want-to-bother-with-dating/