First, I want to say that yes, a big issue
is that we now lead more sedentary lifestyles and there is an overabundance of unhealthy foods in society. HOWEVER, I'd like to point out something that many people do not know. I even made some pretty MS Paint pictures to illustrate my point.
Here you are, you're a normal weight. This is a sample of your fat cells. They're just being cool and chillaxing.
Oops! You got fat. Maybe you're getting a bit older and your metabolism has slowed down. Perhaps you just tend to overdo it on the desserts while you just have a desk job where you sit on your butt for hours. You aren't a teenager anymore, so you aren't outside playing football or running on the track team anymore. Look what happened. Your fat cells increased in size. However, they can only increase in size so much until new cells must be created.
You decide that you really need to get in shape, so you do the right thing. With proper diet and exercise, you did it! You're back to your original weight. However, something is different. If you compare this image to the first, you'll notice that your fat cells are taking up the same amount of area (or volume IRL) as in the first picture. However, you still have the same number of fat cells as when you were overweight! In order for you to be the same weight you were originally, your fat cells are now a bit more "deflated" that what you started with. Also, from what I understand (this might be incorrect), this little bastards don't like their small size. They can very easily balloon back up again.
No matter how much exercise you do, your body will not rid itself of these extra fat cells.
The number of fat cells you have can only increase (I'm not counting liposuction or if you get a limb hacked off).
So you see...if you were never fat to begin with, you are at an advantage. However, it is much harder to remain at a normal weight if you were once fat. This is especially true if the weight is lost quickly, as on the show Biggest Loser. If you notice, many of the people on that show end up gaining the weight back. I don't claim to be an expert on the subject, but for some reason it's easier to keep off weight if you lose it slowly over time.
It's a shame more people don't know this.