There are different opinions on this, but the one that makes the most sense to me is evolution: A long time ago, the human race went in for job specialisation, the females were considered more precious as they were the ones who could give birth, and were needed for bringing up the children as well, so the men took on the more dangerous job of hunting, while women took on the relatively safer job of food-gathering and 'home-making'. The males took on more risk, the females more responsibility, and gradually the bodies of both evolved to meet the needs of their specific job, and they depended on the other for what their own body couldn't do in order to survive. Survival = ensuring that your DNA continues to swim in the gene pool, and in such a harsh environment, humans had to have as many children as possible to increase the chances of some of them making it to adulthood. Women developed the ability to do fine work at the expense of physical strength, and became dependent on the male for protection, not just hers, but much more importantly, her children as well. In order to achieve this, she would have to have all her children from the same father, to give him the maximum incentive to protect all her children, and she was usually attracted to the male she considered the most resourceful to achieve this.
The males couldn't follow this approach as a hunter going out to hunt didn't know if he would return. To increase the chances of his genes surviving, the male would try to father children from many women at the same time, rather than from one woman over a period of time, as time was something he didn't have. While this can now be interpreted to show that women are committed in relationships while men are promiscuous, the truth is that both did what they had to do to survive, within the constraints of the role they played. Therefore men were always on the lookout for a female to mate with, and women in turn developed prominent breasts to signal that they were fertile females. They had many other ways to achieve this, but breasts were the easiest ways for a male to identify a female from a distance, and a woman with bigger breasts could be identified from an even longer distance as a female, increasing her chances of finding a suitable mate. Therefore we men now have the logic that if breasts = woman, then bigger breasts = more of a woman. After a million years of evolution, this is now hard coded in our brains, and even the most committed and loving of men will constantly look at other women, particularly parts of a woman that indicate fertility, even if they don’t take it any further. This throwback to early times manifests itself in other ways as well, males indulge in the most stupid (from a female point of view) and dangerous sports and stunts, or spend hours hunting down and blowing up aliens in video games, as an outlet for their hunting and risk taking instincts. In some ways, the modern world is more frustrating for men than women, as women still do what they have evolved to do, their jobs and careers haven’t stopped them from being home and baby makers, but more and more men are being trapped in cubicles as paper pushers, the once fearless hunters now reduced to worrying if Windows will crash today or not.
If you are interested in more information on this subject, you could try watching the excellent 6-part Desmond Morris documentary called ‘The Human Sexes’ which was aired on Discovery, and can now be found on the torrent network.