Usually it's lifestyle, eating habits etc.
The big mistake alot of people make is not eating often enough (believe it or not). There are valid reasons for why grazing (eating constantly - though not over-eating) will keep a person thin. It's this whole feast/famine cycle. The body reacts to it by storing fat to tide it through the day. Eating 6 times a day but with meals of half the size can do wonders, because the body never feels it's starving enough to need to store fat. Throw in a low-fat cookbook, and excercise even once a week for 1/2 hour, and you get a thin person. Eventually...