It's well known that the human brain is primed to find foods with high energy content delicious. The generally accepted model is that, because sources of sugar and fat were so scarce early on in our evolution that when we come across it we just can't help guzzling it down until we're sick and that's the reason why we're all fat. Stupid brains, am I right?
Well, not quite. Our brains are much smarter than we give them credit for. Even when presented with a source of sugar or fat, we, and many other animals, will still only eat as much as we need and then stop. Just think about it, you wouldn't just sit and eat a stick of butter or a bag of sugar for pleasure.
The trouble comes when we mix sugar and fat together, that's when our self control starts to crumble. This is a combination that can't be found in nature and, as such, our brains aren't really equipped to deal with it.
Fat and sugar, mixed together in a rough 50/50 split, produces an effect in the body similar to cocaine or alcohol addiction. Incidentally, the mixture of 50% fat and 50% sugar is basically the recipe for New York cheesecake. Or a glazed doughnut. Or ice cream. Or any other sweet treat that is totally delicious.
Your brain evolved to handle either fat or sugar at once, but once we started messing around with our diets the poor thing became all confused and that, in a nutshell, is why you're fat.