Think about it
We protect that which is ours. We cling to our groups of friends, not inviting new persons in fear of them being better liked than our selves. So is mankind.
I really hate this. And I don't understand it. I always ask around if my friends want to join me and my other friends I we are doing something fun. It's true; the more the merrier. And once you're and adult it's also a great way of managing to see more than a few friends, as time is fleeting.