Hiya there!
Well, for one, corporate greed. In the media, like TV, movies, artists, etc., are usually very physically attractive in most people's opinion. This can lead to pressure on the rest of society to live up to the extremely high standards, as well as many self-depreciating thoughts about how oneself isn't good enough. Actually, this has been happening for hundreds, if not thousands of years! Distribution of magazines and books have put forth what one must attain. I've attached a picture of a corset ad from circa 1850.
In ancient cultures, like Sparta, being physically active and fit was necessary to fit into society, especially for men (which grew up to be strong, noble soldiers). A huge emphasis on being the greatest was placed on everyone.
But, being attractive and being fit are two different things. Having an active lifestyle can improve your life in countless ways, from improving your mood to lowering your risk of obtaining certain diseases.
