I'd say there are two forces at work here:
1) Expectation.
The long and short of it is that alterations to your body signal "effort made". Because the person the change is "expected" from is "not up to the highest (unrealistic) standards". Said standards are often (but not always) the result of somebody 'not thinking critically' - or just 'not thinking at all'.
An easy example would be the fashion industry. _Obviously_ somebody looking to sell extremely exclusive and unique clothes has a lot of money on the line, so they will always try to have those clothes be presented by the most attractive person they can find.
Having seen a few of the ladies who strike some a poses for Calvin Klein & Co., one wouldn't necessarily say to themselves "I could date her..." - but personally it think it "likely" raises the bar for how "a perfectly beautiful person" would look like. Or as others sometimes put it "a perfect 10". Add to that models in regular advertising, actors in Hollywood movies, etc... Suddenly somebody's inner rating process looks a lot different.
But when somebody points out how the person making remarks about another person's appearance looks like, they will immediately try to point out their "inner beauty" and "character". ;p
2) The "Wisdom" of the Crowd.
Essentially: As a lot of people and they will tell you a bunch of different things. How a healthy/beautiful body should look like is completely subjective. There will always be multiple demographics displeased with something, no matter what it is. Even if it's something your sample group previously collectively agreed on they all liked.
To quote James Stephanie Sterling:
"Attempting to keep 'everyone' in your audience happy is a path towards the steady erosion of your own mind."
In the end, you'll always disappoint 'someone', so you might as well get that tattoo you always wanted and do exactly as much or as little work on your body as you feel good with. ;3
I know I overdid the response, but this was fun to write. :)
Scudlez
One reason...society.
Ayumzy
RIGHT?! its annoying