My boyfriend is obsessed with his ex?

My boyfriend and I have been together for years (it’s a happy and healthy relationship). Him and his ex (of a couple of years ago, he left her for me because she treated him really badly) used to love each other a lot, until they both messed up and screwed each other over. They hate each other with a passion now, and in the hallway they always insult each other and can’t be in the same room together. Why do they still care enough to do this? His ex is also in a happy relationship, and she doesn’t go out of her way to make him mad, she only gets mad if he tries to make her mad.
My boyfriend is obsessed with his ex?
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