People tend to be friends with people who enjoy the same sort of things, if you're a gym rat you're hardly likely to hang out with someone whose idea of exercise is going to the kitchen for more popcorn. People's metabolism's do differ so a group of friends can produce a few that don't fit in with the general picture.
Actually this wasn't true for me, although it is becoming true more now.
I was always an active person, but most if not all of my friends were extremely sedentary. Most had chronic health problems and couldn't get out and do much. Also, growing up, I didn't really have friends, and where I lived, girls simply were not active. I was, but I was given a lot of flak for it. I was very slim in my childhood and teens, but gained a lot of weight, unfortunately, after that.
Now, I have more active friends and am hoping that some of their habits rub off on me, as unfortunately I have picked up too many of the habits of my sedentary friends...
I also know I don't eat right, but I don't like to cook, so I just eat out or have something I can pop in the microwave.