Why do mothers always have issues with their sons girlfriend/wives?

I'm in a situation where my ex's mom used to be kind to me and invite me to all sorts of places, but ever since we broke up she used that as an opportunity and begged me to stay away from her son. She has continued to express that hatred and I can't understand why when I have never done anything to offend her.
Why do mothers always have issues with their sons girlfriend/wives?
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