Ghrelin excess - makes us eat more, we grow fatter.
Insulin resistance - excess insulin makes us fatter, lean tissue is insulin resistant, and fat tissue remains insulin sensitive.
Fat tissue glucose sink - fat tissue is protective, it absorbs excess glucose, turns it into fat, fat tissue grows bigger.
Insulin-induced lipohypertrophy - caused by chronic hyperinsulinemia, cannot be reversed by diet alone, originally discovered in diabetes type 1 where we inject insulin in the same spot for years, specifically called insulin-induced lipodystrophy.
Leptin deficiency - leptin causes us to stop eating, absence of leptin allows us to continue to eat, we eat more, grow fatter.
Hypothyroidism - metabolism drops, we grow fatter.
Semi-starvation - metabolism drops, stays there (subborn metabolism), we grow fatter. The opposite is hyperactive metabolism.
Various vitamin and mineral deficiencies - all involved in some way in regulation of glucose and fat metabolism, we grow fatter.
Fatty liver - leads to liver insulin resistance, excess insulin, excess blood glucose, we grow fatter, diabetic type 2.
Inflammation - caused by fat tissue secretions (necro-something factor or whatever), caused by fat tissue excess growth, leads to further fat tissue excess growth.
Fructose - causes or leads to fatty liver, and so forth with insulin resistance and hyperinsulinemia and all that jazz, we grow fatter.
Ketones shunt hunger - carbs stimulate insulin, shuts down ketogenesis, absence of ketones, we eat more, we grow fatter. A well-accepted principle of low-carb, due to simultaneous higher blood ketones and lower hunger.
Metabolic advantage - low-carb diet causes greater energy expenditure than high-carb diet, balance determines fatter/leaner.
Thermic effect of food - foods cheap to digest and absorb make us fatter, foods expensive to digest and absorb make us leaner.
Yo-yo dieting - leads to low metabolism, we grow fatter.
Genetic predisposition - our parents were fat, that's why we're fat.
Generational epigenetics - similar to genetic predisposition, our parents were fat because of some environmental agent that altered their fat tissue and/or regulation thereof in some way, we inherited this alteration, that's why we're fat.
Pica - a form of eating disorder where we are compelled to eat something that isn't normally nutritious, a variation of pica where we are compelled to eat something that is caloric and/or stimulates insulin, we grow fatter.
Weak will, lack of willpower - we eat too much cuz we just can't help ourselves, we're morally defective.
We're already fat - we're heavier, less inclined to be active, become sedentary, we grow even fatter.
Age-related metabolism drop - the older we get, the lower our metabolism drops, we grow fatter.
Opiod peptides derived from food proteins (wheat gluten and alpha casein) - we get addicted, eat more, grow fatter.
Abdominal pressure on pancreas - high food volume increases abdominal pressure, pushes on pancreas, higher insulin secretion, we grow fatter.
Lipostat - something disrupts normal homeostasis of fat mass, we grow fatter.
Sedentary-induced insulin-resistance - activity leads to insulin sensitivity, lack of activity leads to insulin resistance, we grow fatter.
Stress/comfort foods - leads to eating comfort foods, leads to eating more, we grow fatter.
Self-image problem/comfort foods - any problem with self-image such as lack of confidence, failure or physical flaw, leads to comfort foods, leads to eating more, we grow fatter, leads to further self-image problem, etc.
Poverty - leads to eating empty calorie foods, we grow fatter.
Exertion - we work hard, get hungry, eat more, grow fatter.
Growth hormone deficiency - GH causes fat tissue to shrink and/or remain normal size, absence of GH allows fat tissue to grow bigger. GH also affects all other tissues including lean tissue and muscle, leading to change in metabolism and so forth.
Estrogen excess/estrogenic and/or estrogen-like foodstuff (soy anyone?) - women are naturally fatter than men by about 50% (for example, 15% bodyweight vs 10% bodyweight), excess estrogen in men causes gender modification (turns them into women, or gives them female traits), fat tissue redistribution more like a woman's, also modifies muscle distribution more like a woman's.
I had a link to a page that listed a bunch of drugs and medications that have the ability to make us fatter or leaner, can't find it at the moment, but this would be called something like drug-induced appetite/hyperphagia and/or drug-induced excess fat accumulation.