what is the scientific reason for humans having large, permanent breasts? As far as I know humans are the only animals that have breasts from puberty onwards while all other animals only have then when lactating. I don't know if there are any practical benefits to having them because if they were useful then you would see them all over the animal kingdom.
if the answer is simply "because it's sexy", how did it become sexy? I imagine a pair of big milkies would be a sign a woman is already busy with a baby.