I don't think we should say that all girls feel shame when they develop breasts. Developing breasts is the start of being subjected to the male gaze for many girls, and that's upsetting and can make a girl feel ashamed, but a lot of adolescent girls feel good when they first start developing breasts. They feel more grownup and beautiful.

I know I did. When I started to develop breasts, I felt beautiful. I looked more like my mother, who I thought was very beautiful.

The upsetting feelings came later based on how others treated me, and even how others talked about their own breasts. My mother talked about her breasts (and by extension, all breasts) the way you describe in your comment and it was deeply confusing and disturbing to me as a young girl. Feeling happy about my breasts but being told girls and women are ashamed of their breasts was not good for me.