Since humans are one of the very rare animals that have sex face-to-face, men have had more opportunities to admire a female's breasts during sex. Over time, breasts became associated with sex itself. And we all know men love that.
Back when our ancestors lived in caves and had little access to food, a woman with more fat on her body was more likely to successfully carry out a pregnancy. She had more energy to pass on to her baby. So women with more fat reserves became more attractive. And since breasts are primarily made of fat, and are also a good symbol of the ability to store fat, they can grab the attention of any old caveman.
No matter where you are in the world there is one thing for certain, men love a woman with an hourglass figure. This is because women with this body shape are less likely to get a variety of diseases. And you can't get that classic curve without the top part of the hourglass.
Men love busty women, sure. But breasts are not loved in the same way across the world. There are cultural differences. "In some African cultures, for example, women don't cover their breasts, and men don't seem to find them so, shall we say, titillating." says anthropologist Fran Mascia-Lees.
When a woman is at her prime, her most fertile, her breasts will usually become larger and firmer. So think of large breasts as a natural fake out. Men will instinctively gravitate towards a voluptuous woman because she appears to be more fertile.