Our brains evolved in a time when food was more scarce and the supply was unpredictable, your brain wants you to eat and put on weight in case there's a famine. Once you put on weight the brain records a new 'set point', when you deliberately lose weight with less food intake and more exercise your brain is not happy, it thinks it's being starved.
Here's just one piece of research about what happens in the brain when you diet - neurons in the brain freak out and start eating themselves, a process called autophagy which triggers hunger signals to get you to eat.
http://www.sciencedaily.com/releases...0802125546.htm
Not impossible to lose and keep weight off but the odds are against you and it's not all just will power. And it gets harder as you age, metabolism slows down with age.
The aim should be obesity prevention, this childhood obesity epidemic is sad and preventable but parenting, lifestyle and technology being what they are it's not going to get better. College age is another high risk period, it's usually the first time for most people that they put on extra weight, was for me - junk food starchy diet, beer and alcohol and less physically active than they were as teenagers.
Let's not forget that there are people who through no fault of their own are obese, there were always one or two 'fat kids' in every class in elementary school, genetically screwed just the same as there were kids who looked unhealthily thin who no matter what they do and eat can't put on weight.
Like the article says, we are more biological machines than anything else.