I can't answer why they are attractive, but I can answer what makes them attractive. Breasts are really attractive because they show off a woman's curves greatly, larger breasts show off more curves (now i'm not into the huge busts sizes but hey it's just an example). Not only that I believe it has something to do with us since our infancy, since you were "probably" breasts fed, to feed on them again is probably going to be pretty natural. I would even go as far as calling it biological.

Now if someone could explain to me what makes an ass is attractive (is it muscular? toned? shapely?), that'd be great to know