Insecurity. At least in my case. When I was in college I lost a ton of weight and was borderline anorexic. At 104 pounds, with my abdomen a full inch inside my hipbones, I looked in a mirror and saw a fat girl. I look at pictures from that time and I was skinny as a rail. My brother yelled at me to eat and I just couldn't. BUT, I was very insecure, thinking I was obese, and would often comment on it. Fishing for compliments? Perhaps. But inside my mind I was all twisted up with a very confused and inaccurate body image. I've gotten over it, and now have weight to lose. But I try to remember how I felt then, and feel compassion for others. We never know what's going on in their heads. Does it hurt others? Yes. But try to remember that it isn't personal. When I was that way, I never saw others as fat. Only me. A tiny part of my rational brain could see that folks who were much larger than I looked great. But inside my head it was, "yeah, but *I* look hideous." Weird, twisted, and sad, in hindsight.
Wheresoever you go, go with all your heart. -Confucius