Oh, that kind of habit. The good kind. It can take much less than 1 day to form a bad habit. I wonder how that asymmetry really is - were ancient humans less exposed to addictive substances and had no need to evolve a way to stop that? I have no idea what I'm talking about so, I'll end the comment here.
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.
Maybe it's still a honeymoon thing for me but I've been trying out keto diet for few weeks and, anecdotally, now whenever I eat carbs I get a weird high. If you told me that people started farming to do it consistently, I'd believe you. Similarly, if I avoid carbs at all cost even for a day, by night I feel so bad that I must have some. Point is that as far as I can tell eating carbs is a habit/addiction as well, but much good has come from maintaining it.
I don't remember where I got this quote, but it makes sense to me: "Grains are a cheap caloric source that's allowed for the growth of tightly packed, heavily populated cities. Grains are good for civilizations. They are bad for individuals."
