Overeating, overdrinking, oversmoking.... I see it as a matter of will of power and self-discipline. In any case, people can always have a huge bowl of romain lettuce with vegetables and tuna or lean chicken with a fat-free dressing instead of Big Mac with fries. People don't end up obese when their extensive diet is based on vegetables and lean meat.
My husband is a vegan. I have tried to adopt that lifestyle, but there are cravings and psycological problems that get in the way. I can go days with just fruit and veggies, beans and nuts, but then I crave bread and baked goods, chocolate and chicken. My husband doesn't have these cravings. We are differently wired in this respect. I think that many of us who are obese have the same difficulty. It goes beyond will power and discipline.