It has a lot to do with evolution and biology.
Animals don't have access to a grocery store available at all times of the day for their eating pleasure. Humans do today. If you put a bowl of cat food down that is always full, you will end up with a fat cat.
That is how mammals evolved. We have evolved (from the past) with having large infrequent meals. (Like the lion eating the wildebeest) So that is what the body expects and is wired. Today, we are told we should have 6 snacks a day between 3 meals. And as far as evolution is concerned, is a very bad idea.
This has everything to do with
http://en.wikipedia.org/wiki/Leptin
Although leptin reduces appetite as a circulating signal, obese individuals generally exhibit an unusually high circulating concentration of leptin.[69] These people are said to be resistant to the effects of leptin, in much the same way that people with type 2 diabetes are resistant to the effects of insulin. The sustained high concentrations of leptin from the enlarged adipose stores implies leptin desensitization. The pathway of leptin control in obese people might be flawed at some point, so the body does not adequately receive the satiety feeling subsequent to eating.
So in other words. The more frequent you eat, the more you mess up your body's hormones. Your body becomes immune to the "I'm full" signal even if you eat 6 snacks a day and 3 meals. Since your body has become resistant to the "I'm full" it stays in starvation mode and stores every calorie it consumes when it should be giving you energy. Strange that the fat person is always hungry and never has energy and is tired all the time? It's because they are leptin resistant. Makes sense why, right?
This will happen with animals if they are allowed access to food indefinitely. Humans should really only be eating as needed, none of this 3 times per day with 6 snacks garbage you see all the time. If your body is working properly you shouldn't need more than 2 meals per day (and no snacks)...
I've followed the leptin diet (which is a diet based on how frequent you eat) 6 months later, I had no issues with 2 meals per day, and had lost 60 lbs because I lost the leptin desensitization that happened when I was sipping sugary drinks all day, etc. (I still drank sugary drinks, I just only had them with meals, so this diet has nothing to do with calorie intake, and it just requires you to only eat/drink at meal time)