This is truly the million dollar question. Well, I'm ready to make a million dollars, so here goes. Our bodies were forged by the pressure of natural selection over millions of years. The primary objective is to survive to reproductive age and produce offspring. The biggest challenge in achieving this objective is obtaining food. Life is basically a game of converting calories into kids. In this game the value of a particular food is calculated by the following equation.
food value = (calories provided by food - calories used to acquire food) / time spent
We are evolutionarily programmed to desire the highest value foods because that gives us the highest chance for survival. There's a trick, though. Our bodies don't directly measure calories, so how are we programmed to seek out the highest value foods? We can't measure calories, but we can detect fat, carbohydrates, protein, sugar, and salt. These chemicals trigger a dopamine (the feel-good stuff) response in our brain. In other words, our brain rewards us for obtaining foods high in fat, sugar, and salt. That dopamine release cements neural pathways in our brain that ingrain the habit of obtaining that food. That's how our brain trains us to be experts in acquiring high value foods.
Back in primal times, food was generally hard to come by, meaning it required time and energy (calories) to acquire it. Also, food was in it's natural whole form, meaning it didn't have the caloric density of today's processed foods. Fast forward to today and we have access to processed foods that score higher in food value (refer back to the formula above) than anything that existed back in ancient times. Take a primal man and drop him into today's grocery store, and you can imagine the free-for-all that would occur in aisle 7. Now realize that we are still the same biological creature as that primal man, and you can see why we eat too much. Evolution didn't build much of a braking mechanism into our circuitry because it never conceived of the possibility that foods so high in caloric value would be so readily available. And THAT is why we are fat.
The good news is that there isn't anything wrong with us. There is no reason to feel guilt or shame. We're simply expressing the talents that evolution bestowed upon us. The problem is one of environment. Acknowledging this removes the emotional component and allows you to focus on strategies to surround yourself with quality whole foods... and keep processed foods as far away as possible.