Eating. The bottom line is that if you take in more calories than you expend, your body will save it as fat. Bad food choices coupled with a lack of exercise are probably the causes. Thin people eat healthy food [fruits, vegetables, lean meats and dairy] and they are active. Obese people are, in general, not active and have poor eating habits.
Sugars bad? I will amend my previous answer: there is some new research [as seen on 60 min last week] that simple sugars are handled differently by the body than more complex carbs or sugars with fiber such as fruits. The liver may become overwhelmed with an easily absorbed large simple sugar load and faster storage to fat may occur even with the same calorie load as a balanced meal. Interesting stuff!
Unknown. A lot of research is being done, but not sure. People do eat a lot of calorie dense foods and have less-active jobs. Cigarettes kept some people artificially thin. Plus, recent studies show that normal-sized people may have dangerous obesity-related diseases and vice-versa. My bet is on eating too much processed food, especially chips and snacks. But i eagerly await an answer also.
Intake > expenditure. Certainly the food industry plays a role in providing poor quality food which is cheap and in abundance. But ultimately choosing poor lifestyle is to blame. We eat fast food, quick meals, food in boxes, lead sedentary lives driving around in cars. We eat gargantuan portions frequently in restaurants, drink calorie laden soda and sweet tea. Poor lifestyle choices as a nation are the reason!