So, so wrong, unfortunately, everybody here.
Especially Mt.FishNoob but I never know whether I can take his posts seriously anyway.
(But we're wrong in part because nobody is right or quite close, even — obesity and nutrition research are so deeply in their infancy yet.)
Working out like crazy does not
guarantee fat loss, as many studies have demonstrated. Furthermore, training specialists are well aware that fat loss programs can only be cycled — they don't work continuously. The idea that sugar makes you fat is simply unsupported by the data. Defining the word "sugar" helps, but in that context we normally mean that bad sugar = fructose. And fructose only causes problems within specific contexts, with respect to fat gain/loss, and also is an indirect problem (per leptin, reward, etc).
Roggo is completely misguided here. The "morality" hypothesis of obesity has been long discounted and, indeed, people are "made" fat. This is through poor education and ubiquitous, instantaneous availability of bad food. I don't mean to give the wrong idea here. The sane reality is that eating well yields better health. In this context, eating well means to limit unnatural fructose (i.e. non-fruit), processed oils (e.g. almost all vegetable oils and refined oils), meta-foods (e.g. high-density, high-reward), and focus on real, whole foods based around high-quality animal products.
Leanness ≠ health, but health often leads to leanness. Skinny obesity certainly exists, as a metabolic disorder. This is especially true in younger people eating extremely unhealthy diets (as Mt.FishNoob paints himself). The issue is that strong genetics fight powerfully against excessive fat gain. With age, these forces fail, and thus you have, "Wait 'til you're 35."
Nutrition and obesity are very poorly understood. Try to not make any firm claims to anything. All we know, for now, is that the healthiest people typically eat real, whole foods, based around good animal products (fatty, wild/pastured meats, eggs, butters, bones, etc), fruits, vegetables, and whole starches. Typically, there is a seasonal variety in these diets, with a regular intake of fermented foods. Follow their example: eat well, walk a lot, and engage semi-regular, short bursts of maximal effort exercise.