Everyone is at some point.
Insecurities, mostly brought on by society's high expectations.
Popculture, the nature of women - sociobiology - they want to attract a mate, annnnnd ****-culture and stuff amps up expectations because now more than ever men and women see pictures of "perfect" toned, skinny, shiny, plastic women near naked if not totally naked.
Men, and other people.
Cultural pressure to conform to an unattainable standard.