That's what I say! I believe for 99% of people it's as simple as you eat too many calories and you don't exercise enough to burn them off!
I don't care what kind of calories they are; fat, sugar, carbos... they're all calories. Although, I do believe in eating a healthy diet with mostly low fat foods and plenty of fruits and vegetables for your own health. You can still clog your arteries whether you're thin or not.
I believe the number one thing to staying healthy and unfat is regular exercise (aerobic activity and weight training)! A decent diet and proper rest don't hurt either.
I eat everything... mostly a balanced diet with tons of carbos, but also sugar, chips, cookies, BEER, etc. I'm in great shape because I've run 40 miles per week and done weight training for the past 30 years. I'm 45, can still run a mile under 4:50 at the present time (Trying to get back to sub 4:40) and can bench press 300 lbs. I weigh 170ish at around 6'. My brothers on the other hand are overweight and in BAD shape physically (IMO)! One is 6' 215-220 and the other is 5' 11" 250-260! They eat less than me, but do 0-1% exercise a day! Same genes and everything!
Lifestyle is 95% of everyone's problems and that's the TRUTH!! You heard it first or second!