Desmond Morris' theory is always good for a laugh or to break the ice at parties:
Chimpanzee (and, so is postulated, early human) females are only fertile for a certain part of their cycle. Unlike humans, dolphins and bonobos, who copulate at the drop of a hat, to conserve energy chimps only mate during the female's estrus, which is signalled by blood engorging the female's posterior - the male sees the big swollen ass, a switch flips in his head, and 4.3 seconds later - the deed is done.
Whether they're ruminating on the forest floor or brachiating (juss kidding - I don't believe chimps brachiate) they tend to roam around on all fours, making a receptively-inflated posterior quite a spectacle. It is thought that this visibility is one of the things pre-humans gave up as they adopted a bipedal stance - a combination of the change in orientation and stronger upper-leg muscles rendered the swollen area invisible between a woman's legs. So, it is thought, a more-visible analogue for them had to be developed.
Here's where we get to the breasts. Apes of all kinds (and indeed all mammals - it's what makes us mammals!) breast-feed their young and have sufficient equipment for the process, yet they don't have the great protruding hooters of the race Dolly Parton can be said to represent. It is supposed that both non-estrus sexual activity and pronounced breasts were successful evolutionary traits at getting around the males' disinterest at lack-of-genital-flares.
To succinctly summarize: we like big boobs because they remind the chimp part of our brain of a big swollen bum. Any questions?