I just lost 14 pounds. However, the instincts around body homeostasis are very, very strong. I don't think it's reasonable to presume that it's all self sabotage.
Btw, I lost 20 pounds in the last 6 months, so I know what it's like. That said, once you make it a habit, it's not that bad. The hardest thing with food is avoiding "just one cigarette". Just takes a few days of crappy eating / not exercising for your old habits to take over again. You really have to watch out for that.
This is a big part of the dynamic of alcoholism, drug addiction, bad relationships, and stupid dogmatic ideologies.
I bet it plays into working life and the various "geek scenes".