Why do exes try and rub their new partners in your face?

Anonymous
I recently bumped into my ex after no contact for about a month. I was with a friend we both knew. She started talking to him and kind of alienated me from the convo, I just acted like it didn't bother me and tried to get involved with the convo. I make a comment on something our friend says and she blows it off and makes a comment about her new boyfriend going somewhere with her. I also acted like I wasn't bothered by that even though it was the same guy she left me for. Then my friend noticed she was being awkward so he ended the convo. Why did she ignore me even though I was nice and acted like I was unbothered by her being immature and alienating me? She couldn't even look at me. And why did she even mention her boyfriend? Just to get a reaction out of me?
Why do exes try and rub their new partners in your face?
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