Just to address the "calories in and calories out" thing, the mental health side of things notwithstanding:
That view of the body as some kind of perfect and homogenous conversion system is incorrect. Obviously, because otherwise everyone would be the same. The whole makeup of the body determines how incoming "calories" (not all calories are equal - it's a buzzphrase, but true) are processed and converted - metabolism and hormones are not some sort of static process, but an ever-evolving and shifting system produced by genetic influences and environmental development. Cumulative effects over the lifetime, along with the genetic + developmental startpoint, lead to the very different endpoints in the "conversion engine". This is why different drugs you are on affect your weight gain or loss, this is why insulin resistance affects your weight, this is why some people have that "natural advantage" and don't accumulate metabolic damage (for want of a better phrase) which, over time, fecks up your ability to process food as efficiently as others, death-by-a-thousand-cuts style. You can work against it with exercise and so on, but it is always more of an uphill struggle for some than others.