I don't know...

There are studies done that show as you get older you naturally lose friends and the circle of friends you have get tighter. This makes sense because in elementary/highschool/college you are exposed to a lot of different people on a daily basis so its easier to forge friendships.

It may be that people who have MI are predisposed to be higher in neuroticism which makes them worry more, which makes it harder to become friends with people. I am going to ask my personality professor about this to see what he says.