Tara Parker-Pope of The New York Times has written an excellent summary of research on set point theory, that is, the body's tendency to defend a higher weight after weight loss. And some of us may just have to work harder to maintain weight loss because of other genetic factors.
This research can be used as an excuse for weight regain - "What's the use?" or "My genes made me do it" - or as information that can help us forgive ourselves for sometimes falling short of our goals. Of course, I recommend it as a source of compassion, not an excuse for poor eating habits.
Tara Parker-Pope's article
The research also supports the idea that gastric bypass surgery resets the set point that the body will defend going forward. Surgery is not a simple engineering change. Rather, it starts a complex biochemical process that supports weight loss until the new set point is reached.