> What if it's not. What if people just overeat because life is boring, and food tastes good, and we all have differing levels of willpower & education on nutrition?

Then we need to explain why this suddenly became a problem in the 1970s. Life is more boring now? We all lost our willpower? People understood nutrition better before that? Something has changed, either in humanity or the environment. My bet is the environment.

Was there enough extremely cheap and tasty food before the 1970s?

There is a specific part of the hypothalamus called the arcuate nucleus. It lacks a blood brain barrier, because the neurons there have a special function to sense blood-born hormones like insulin and leptin. When sensed, the brain interprets this as a "full tank of gas" and suppresses appetite while increasing metabolism.

Since theres no blood brain barrier here, these neurons get damaged easily due to things in the blood. When that happens, they don't see leptin anymore, and obesity develops. Fructose and high fat diets have both been shown to cause inflammation here. (hypothalamic gliosis)

heres a random selection of many papers http://diabetes.diabetesjournals.org/content/65/3/551.figure...

We also know that certain viruses show up in obese people (adenovirus36). As the field develops, I would not be surprised if labs demonstrate both viral infection of the hypothalamic ARC and damage by endocrine disruptors there.