Apparently, the common pattern of people in any society is to have two important childhood friends, two significant adult friends, and two doctors. Typically, there are two powerful sexual partners who eclipse the others. Most commonly, you only fall in love once, and there is one member of your family whom you love above all others. The number of significant personal relationships is remarkably similar for everyone, regardless of their location, sophistication, or culture ... And once these slots are filled, they're filled forever.
Richard Koch, The 80/20 Principle: The Secret of Achieving More With Less (1998), p. 177
From Josh Sowin's ever-thought-provoking Fire & Knowledge