It's natural instinct. Back in primitive times, people with large boobs were learned as better at raising children, and that eventually came down to us. Also, large asses are arousing because the people who had them tended to be more athletic and better at defending their families, so that also came down to us.
In addition, we tend to notice things that are out of symmetry. We like babies because they're small, we notice big or small cars on the road. Boobs and butts break the natural symmetry of the body, so they are more eye-catching.
