It's ye ol' confirmation bias. If you hang around the same
  people who think similarly to you, you all tend to have the same
  knowledge and beliefs, more or less.

  But the more diverse your friends, the more likely you are to
  come across someone who will continually surprise you with a
  completely novel outlook.

  I try to keep friends of different types, although I need to
  expand more, for that reason: They may appear ignorant from my
  point of view at first... but think of this: It goes both ways:
  THEY likely have information that I _don't_ have and that _I'm _
  ignorant about.

  There something in the theory of 'degrees of separation' about
  that. The people furthest away from your immediate friend
  circles - the 'friend of a friend' (or friend of a friend of a
  friend of a friend...) is where your opportunity for growth
  lies.