I don't know why the ass is appealing to some. For me, it's the face and the abs. I just don't like seeing any extra fat whatsoever on either (though I have it in spades in both areas). Athleticism is sexy. Not really sure why, but I do like sports. In particular women's soccer and gymnastics.
"For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring." - Carl Sagan