Your individual basal metabolic rate - the way your body turns food into energy - is unique to you. Some folks naturally have a fast metabolism and have little to no trouble losing weight. Others, while they may have been born with a powerful engine, may have damaged theirs with years of starvation dieting without realizing it. This is most likely the reason they have trouble shedding pounds now and not a naturally slow metabolism.
Recent studies have shown that your metabolic rate is not a fixed entity, but a variable. Starvation diets can alter your fat-burning capacity in a negative way.