As a strong believer in Darwin's Theory of Evolution, I believe that the form, structure and function of everything in the body has a purpose. Now, in the case of breasts, if the purpose isn't for natural selection (see my writeup on breasts), then it's quite likely the purpose is for sexual selection.
Sexual cues are complex and intertwined with the vagaries of society. For a good read on this subject, there's The Third Chimpanzee by Jared Diamond, where there are some classic drawings showing relative breast to body size and penis to body size and testes to body size for some primates (chimps, apes, gorillas and humans) and a reasonable explanation for these.
Anyway, my take on why men like women's breasts is that this is the way we have evolved. It is who we are. Breasts indicate to men, at least on a subconscious level, that a woman is fertile -- inasmuch as she is past puberty and that her body can afford to waste energy on developing and carrying around nicely shaped but otherwise unnecessary pieces of fat -- and therefore a potential mate.
After all, the biggest misconception regarding breast size is that bigger equals better ability to lactate or provide milk for offspring. This has no scientific basis. A woman with an A sized cup can nurse just as many babies as one with a DD size (assuming both are healthy and there's nothing wrong with their ability to lactate). The vast majority of what makes up breast tissue in the non-lactating female human is actually adipose tissue (fat) and connective tissue.
On a personal level, I find it highly illogical that men like breasts as much as they do, but that hasn't stopped me so far.
Crow Jane: in a sense, you've got it. Estrus or ovulation in the human female is hidden (even from the said female herself), in stark contrast to what happens in pretty much almost every other animal in the world, where it is usually painfully obvious when the animals are ready for mating or are in heat.
It is arguable that the hidden ovulatory patterns of female humans has led to the situation where the male partner must stay close to his woman, guard her and mate with her on a reasonably regular basis to be reasonably sure that whatever children arise are actually his. This is probably a good thing.