Can I add something before this thread hits 1000? If we're on to analogies, this would be mine.
A large group of friends exists. Within that group, some people inevitably rub some others up the wrong way. Just the way life is really - some people you click with, others you don't. And some have major disagreements/arguments every time they meet.
A dinner party is organised. In the interests of having a small, more intimate chat than the loud, busy chat they normally have, only a certain number are invited. Someone says - hey why don't we invite person A. As it happens, every time person A meets person C they have a violent disagreement, and it's very unpleasant for everyone. So person C says 'well, ok, but I will have to not come in that case.' At which point everyone says 'no, no, you're here, you're coming, we'll leave it as it is.'
Clearly, the friends can't say to person A 'we're having a dinner party, you're not invited, but want you to know'. But person A finds out. And says person B who is at the dinner party 'Either you don't go to the dinner party, or I won't be friends with you any more.' Forgetting that she can still see those friends at her own dinner party, or at toddlers, or at another friend's house, or at the place where they always used to meet. And forgetting also that perhaps sometimes they have had problems with person C.
It's not a perfect analogy, but it's kind of the way I see it - not everyone gets along with everyone else, and in a small group of people, some sort of membership selection is bound to exist. And just because person A is friends with person B, doesn't mean they can't be friends with person C, even if person A doesn't get along with person C at all.
I'm sure you will all pull this post to pieces, and tell me I'm patronising and to f-off (which is what happened last time I posted), but it's my clumsy attempt to explain my thinking on this.