We have not spent the last two hundred thousand years within walking distance of a McDonalds. We have had to work very, very hard to extract even a minimal level of energy (sugar in some form) from our environment.

So our bodies are geared up to use the sugar we can get. They are very efficient. And when we eat large amounts of refined sugar (or white bread), we get a quick kick of energy levels or forefathers could never dream of.

Result: A bit of happiness. Until insulin is released to regulate our the metabolism (digestion) of carbohydrates (sugar). And we have the famous sugar crash. Too much of that and not only are you not very awake and happy, but you could get diabetic.

As a link to the processed food issue - processed food also tends to contain a lot of refined sugars (wether as straight sugar or refined flour) and this is why we are seeing such a massive in adult-onset diabetes amongst fatter westerners.

To get closer to what would have seriously pleased great-great-great-great grand dad, eat carbohydrates that haven't been turned into this rocket fuel of refined sugar. Don't eat white bread, eat dark bread. In fact, this is a great rule of thumb in general: the darker the carbs, the slower it'll release energy and the more energy you'll be able to make use of and you'll avoid the crash.

Glycemicindex.com is a valuable resource to get to grips with how foods energy is released into our bodies.

And here is a great article on High-Fructose Corn Syrup.