Very nice said. Alice's behaviour is selfish and she is trying to force others to accept her subjective point of view (in whatever cost). Why does humans behave like this? I am sure that this kind of behaviour is quite common as you said, but I am consider it as a little bit... pathological. Alice definitelly would not like if others will behave the same to her, so in the end, this behaviour cannot bring her anything positive. What do you think?
I think the underlying cause is that wherever you have a group of people interacting with each other (even when this is only
two people), you have a situation where each participant in the interaction wants different things. The outcomes that you as an individual want are 'right', and those you don't want are 'wrong'.
In this situation, for Alice, the idea of Frank being with Marry is 'wrong'. It doesn't really matter what in her mind the 'right' outcome is, whether she would like to be with Peter but also stay with Frank at the same time, or whether she doesn't want anyone to be with Frank, or whether she just doesn't want her friend to be with him. It's enough for Alice that Frank being with Marry is 'wrong'. She has not reached this definition of 'wrong' through any logical reasoning, it's simply an emotional position. What she then does is to try to seek any reason she can find that might support this conclusion. She then decides that Marry's action is not compatible with the rules of friendship... because this is a reason that supports in her desired conclusion, that Marry+Frank=wrong. Alice's reasoning is superficial and inauthentic. Her 'reason' for Marry's action being bad is not a reason at all, it's simply a mental prop to support a pre-established conclusion.