I was wondering when someone was going to mention the TV issues. What spongemonkey13 said it absolutely the issue. I'm thinking of Patrick (in MMBB4) when he's talking about "wumbo," and he says, "It's first grade, Spongebob."
"There are more calories being consumed than burned off." That's it, right there. Now the specific reasons for why most of us reside in a nation (the US) where this is so often the case is another argument entirely. Speaking on the surface of things, if you consume more calories than you burn off, you gain weight. That simple.
Of course, we likewise have to add in other factors (the slowing of metabolism that tends to happen with age, the unhealthiness of the calories we do consume, little kids having less say in what they eat, etc.)
But most of the previous posters have hit the socio-cultural issues that contribute to the problem. Indeed, we are responsible for what we eat, but we still have to place such actions in the matrix of the sheer availability and convenience of fast food as well as the matter of corporate profit (McD's is out to make a buck, not worry about your health, as the documentary Super-Size Me demonstrated.)
Speaking of Super-Size Me, it is an interesting book and documentary (can't recall if they have the same name.) Again, I've forgotten the title, but there's actually another book out there that refutes the conclusions that guy reaches (the guy who only ate McD's for a month.) I'm not sure what her conclusion was (I'm pretty sure it was a woman who wrote the rebuttal.)
But in the end, I do have to say that I subscribe to what Lil Loco said: "yea, im 320 pounds cuz mcdonalds force-fed burgers down my throat and i had no say it." In the end, what we eat is truly our decision.
