The question in the title to this blog would, at first blush, seem to yield an obvious answer. Think of the fast food industry and its tactics, increasing portion sizes, and of inactive bodies with eyes glued to television and computer screens. Too much food (especially of dubious nutritional value) and too little exercise: too many calories in and too few calories out. The question underscores how individuals of "nondeviant weight" can be pretty quick to judge -- negatively -- those struggling with obesity.
A focus on calories in/calories out is clearly relevant. Encouraging everyone to eat better and to be more physically active is a sound strategy. Such prompting can be especially important for kids so that, from the get go, good health and weight go hand in hand. There's lot that governments can do -- always focused on effectiveness, not just wishful thinking -- to promote better eating/more exercise: from regulating advertising, especially to children, to encouraging physical activity, again, especially of children, for example, incentives built into the tax system such as the Children's Fitness Tax Credit, and community health challenges, such as the campaign recently launched by the Ontario government. There is some recent evidence, in the United States, that obesity rates among young children are falling.
All that said the reasons why people become obese remain complex in many instances.