Why does my friend and her boyfriend constantly talk badly about me behind my back?

My "friend's" boyfriend was always really rude to me, and I could never understand why. He would tell me I'm stupid, make fun of me and things like that in front of her and she would do nothing about it.

I recently found out that a big reason why he dislikes me is because my "friend" actively talks badly about me to him and has done for years, and tries to stir the pot by saying I dislike him also. They were talking behind my back this whole time saying I'm a "bad person" and "stupid" and "not that pretty." What is their problem? I'm only ever nice to them and I've found this out.

Why does my friend and her boyfriend constantly talk badly about me behind my back?
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