If you've ever seen a dog's relationship with food, they can't self-moderate very well. They can get full but a lot of times they will eat until they throw up, especially if it is something delicious. Dogs don't have the intelligence we do but the mechanism is the same. We have the same compulsion.
The reason we over-eat is because our evolution hasn't caught up with our food abundance. We were designed to go long periods of time without eating and then eat when it was available. So our minds are telling us to keep eating when we're full because we may not get another chance to eat.
That's why moderation is hard, and that why things with high fat and sugar content, things that were once rare but highly prized for their energy-giving capabilities but cannot be eaten in abundance without health problems, are delicious.
That's a good point.... and I must say this - When I do eat a lot I have more energy for longer periods of time.
I have always advocated eating a big breakfast when I know I have a lot of work ahead of me. However, even when I know that I don't have a lot of work left to do I still find myself eating larger than normal portions on occasion. I should also mention that I am not overweight in any way shape or form. My take though is that if it's regular food like meat and vegetables, then overeating a little bit is okay. When it comes down to cookies and chocolate bars though.... moderation is definitely required.