My theory is that we don't come from a history of plenty or safety, so, in a pre-historical sense, if something feels good that's supposed to be the signal to keep doing it because it is going to go away and you can't really get too much of it.
We were supposed to overeat because we wouldn't often get a chance. If we could be lazy and survive, then we should be lazy and preserve those calories because a time was going to come when we couldn't. It's the modern world that has all those natural predispositions screwed up. We can enjoy ourselves to excess in ways that we would never be able to in the wild, which then puts us in danger.