Other primates don't have protruding breasts, so there must be something special about our species to account for our males' preference for them. A theory that I read a long time ago was the following: Originally, men pursued women as they pursued game -- from behind; so it was helpful to be attracted by buttocks. Then language was invented, with the consequence that women were pursued primarily from the front, and so corresponding protuberances were selected for. An explanation as charming as that does not have to be true.