Why would your boyfriend tell you horrible things about your appearance?

Just wondering after talking with a friend… Answer like a hypothetical scenario.

Why would someone be absolutely sweet and nice in the beginning, flooding you with love, attention and compliments and later on in the relationship, tell you wonderful compliments one day and the other day they’d tell

“You are pale like a dead”

“Your nose looks like a nose of an animal”

“You should sleep more. Your under eyes are too dark.”

“One of your eye is bigger than the other one.”

“You’re too short.”

“I am tired of seeing your face.”

”I know some exercises to make your breasts bigger.”

Why would he become your boyfriend in the first place if he thought you were this ugly? Why would he point out all the things you dislike in yourself and make sure to criticize them, even though you already know about them?

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“You are too short, you will ruin my tall genes.”
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And then when you broke up with each other, he tells you “I am sorry for everything I’ve done, I loved you but I broke you. My love broke you. If I love you I need to stay away from you. I will always keep hurting you. I am created for destruction, the best thing I can show my love with is going away from you. Thank you for loving me. Losing you will be the biggest regret of my life. You’re the kindest person I know and I will always love you and be here for you if you need me.” - sick, isn’t it
Why would your boyfriend tell you horrible things about your appearance?
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