Once upon a time I spent most of my research time reading about obesity and I remember from back then that people in the field were disagreeing on what causes obesity and how to tackle it.
The concept of a weight "set point" was floating around since people noticed that it is very hard to lose weight in a stable manner, but once a new setpoint was reached and stabilized, the body would defend that setpoint. Similar to temperature regulation (the main topic in my current lab).
The idea is, that something changed in the past 20-30 years that lead to a global change of the setpoint. This thread (and associated book) go through some of the possibilities that caused this change.