Why does my friend talk bad about my butt?

So my friend used to joke and talk about my butt saying it's little and I have a "frosty flakes booty" saying it's flat and hers is big. I'm like why do you care so much. It's starting to bother me. She even compared my looks to hers and just asked a random guy who looks better. I'm like wtf.
Why does my friend talk bad about my butt?
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