My (ex)girlfriend broke up with me a few months ago, and though it didn't end badly as such, for reasons I still don't entirely understand, I bore many negative feelings towards her for a while.
My boyfriend tried to do the typical supportive thing, tried to help when I was sad and even tried to join in on the negativity towards her (not that I ever wanted him to, he just seemed to think it was the 'done' thing).
But, the two of them are like peas in a pod. When I was with her, they always used to joke that they were female and male versions of each other. So at parties, or social gatherings, he would try ignore her to seem supportive of me. Except when they were alone, and he couldn't help falling back into how they always were and chatting. They couldn't help it - they just got on so well.
My negative feelings faded and now we're all friends again. But I did find it quite interesting, that despite my never wanting/asking for it, my boyfriend felt the need to act indifferent towards her for so long.
I suppose it's one of those unspoken societal rules (that often don't work, but so many people seem to just accept), you know like the whole 'make sure to bad mouth your best friends ex with her in conversation - even if you actually get along with them'