One theory I have heard: Yanks & Brits are obsessed with tits because they were mostly bottle-fed.

A far more charming theory I have heard: Humans are the only primates that have prominent female breasts. That is because the invention of language required men to pursue women from the front instead of the rear. Thus, women seeking to make use of the male program of attraction to paired protuberances had to evolve a pair in front.