Breasts in the media are always perky and perfect. They sit plumply out from the chest when their owner is standing and they do not try and hide under her arms when she lays down. These are not real breasts. Real breasts move when you walk. They sag. They sag more after breastfeeding. They sag more as we age. Most women could hold a pencil under them, which when I was young was the sign of bad breasts. Unless your really young, most nipples don't point forward, they aren't always perfectly centered, they are different shapes and sizes and placement.
We do not often see these real breasts in mainstream media, we see these young or artificial or photoshopped breasts and it leaves most women feeling insecure or unsure of how their breast fit in the scheme of things.
This site shows lots of different breasts, none of them fake, all of the breast are perfectly imperfect.
Recently Kate Winslet did a film in which she was topless and she showed her amazing slightly used (she breastfed two kids) breasts in the raw. Sad that it had to make the news because it is so uncommon, but good on her ! hopefully more women will take pride in their slightly used saggy boobs :)