Why do humans have enlarged breasts when other mammals don't?

Excellent question: There has been much speculation and most points to evolution, if you are a believer. While most animals are quadriods and the buttocks is more attractive to most 4 legged animals, as we became upright, some evolution biologists believe the breasts are buttock analogiues when humans face each other. Larger breasts suggest greater health and reproductive capabilities and impart an advantage.