Breaking up is hard to do

... so why do other people make it harder?
How do people react when friends break up? Is the strength of their reaction relative to the strength of their own relationship - or perhaps their past experiences. I suppose it also depends on the circumstances of the breakup and how the parties in the relationship are about it. For example, was it a mutual decision (although I have a feeling that even mutual decisions are actually driven by one more than the other)? Or did one of the partners go off in search of other companionship? Is there a move to divvy up friends or do the friends take sides?