Do any of those friends organise anything for any of their other friend groups?
I get the feeling that some people organise, while most people don't. I haven't seen the situation where a person organises stuff for one group, but not for another group. It always tends to be the same people doing the organising for all their friends. At least that's what I've observed, I'd welcome any other observations.
edit: poor choice of words
That attitude, the some people are leeches, to me is part of the problem. If you go in with the expectation that others owe you something or they're bad people, you're only going to be going down the path of not doing it.
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"Most people have less than the average number of friends", or The Friendship Paradox: