Why is my flat chested friend so bitter about it?

I'm 29 years old and I have a best friend who is also 29 whom i've known for 20 years.

I'm what most would consider busty, 34dd, and she is the other end of the scale at 32aa. She is constantly telling me off for having any cleavage on show.

She never stops moaning about her lack of boobs and somehow feels she requires constant sympathy. I'm getting sick of it, why is she like this and what can i say to make her stop?
Why is my flat chested friend so bitter about it?
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