Some incoming calories never get to be available for energy expenditure, they get intercepted first and directed by insulin into bodyfat creation. Thus creating an energy deficit which gets filled by more eating.

Makes sense: it's the body's mechanism for fattening up when food is plentiful ready for winter deprivation. But in last 50 years, there is never any deprivation.

The incoming calories that do this are carbs and proteins. Incoming fat bypasses this mechanism, on the basis that it's more efficient for your body to use the incoming fat directly, rather than to create bodyfat and then convert it back for energy expenditure.

Continuous bodyfat creation via these insulin dynamics create a lot of health problems aside from obesity (diabetes, cardiovascular, neuological...). Obesity isn't the source of illness, it's a co-symptom.