Many times I'll read a story of someone who lost weight and kept it off. And then they detail their pre weight-loss diet and I think, well of course you were overweight. You were inactive and had a terrible diet (sugary drinks, processed foods, etc). You started getting some exercise and learned a few things about nutrition and the weight fell off.
But then there are others who seem to do everything right and are over weight in spite of that.
For example. I was never overweight as a kid and relatively active. In college and for the start of my career, I stopped being active and my diet was awful (e.g. I thought a large Jamba Juice smoothie and a carrot cake was a healthy breakfast choice). My weight ballooned up to almost 190 lbs (20+ lbs overweight), my blood pressure went up, I started having rosacea.
I started running and fixed my diet. Quickly my weight dropped down to 150 and I've kept it in the 140-150 range for over a decade. The other health issues cleared up as well. But it wasn't hard work for me. Being thin is my natural state if you will, and I had to do everything wrong to stay overweight.
My wife meanwhile continues to struggle with her weight. She's successfully lost weight through extremely diligent calorie counting, but after a year or so she starts to put it back on. I have never counted calories. Our diets are similar (in kind, not quantity of course, she eats much less than me). She is active, but not quite as active as me. So similar diet and life styles, but my weight stays off and hers does not.
Hereditarily, no one in my family is over weight. There is obesity on both sides of her heredity.
And I see this playing out in our kids. My son has an athletic build and will probably never have weight issues. My daughter takes after her mom and it will take a life time of diligence for her to remain at a healthy weight.
It seems that some people are optimized for famine, and some for feast. :-(
Obviously there are a lot of factors involved in the growing obesity crises. But I feel for people who struggle with their weight despite doing all the right things, I really do.