For me, it goes to the fact that I'm not the same person to different people. The fact that I'm willing to share some things with certain people does not imply that I'm willing to share everything with those people.
Very few systems make it easy to manage multiple (and shifting) sharing relationships, so not sharing by default is the easiest thing to manage.
It would be very interesting for me if you might write up a few lines on "why", if you can.