What many of you are ignoring is the science behind it. It's not '"fatalism" it is just plain scientific BIOLOGY. Fat cells do not die, they're not like blood cells who's lifespan is 120 days! That why I corrected the other statement. It's not about me/you/they wanting to balance out the calories needed, it's about the cells WILL balance out the calories. No matter how little YOU eat the cells WILL compensate. That is the science.
In this way, a (always) skinny person can eat a tub of fries and not gain weight, because there are no fat cells lying in wait (pardon the pun)
An overweight person will put on weight with just a half tub of fries, because every single calorie turns into fat. For the skinny person, the calories came in one end and went out the other end!
Not every calorie ingested in every person ends up as fat!
The reality is, if you've been fat in your life, remaining skinny in the future requires 10x more committment than an always skinny person.
Do you think many people have the will power to be 10x as 'in control' as natural skinny people? The only people I know with that amount of self control obsession are anorexics.
It's simply not realistic to expect that from normal behavioured people. Weight loss is eventually regained not through laziness, but through NORMAL human nature.