There are many contributing factors to rising obesity in most developed/western countries.
Chief among them all, the intake of FAR more calories daily, than the body actually needs. People are eating more food daily, than ever before. It's simple science, you're eating more calories than your body can burn.
Now there are other factors. The modern sedentary lifestyle of the west, the decline in people having the ability to cook their own food, an increase in processed food consumption, and a decline in daily physical activity and exercise.
People drive to work, sit in an office, drive home, eat and sleep. Few have time to go to the gym, work out. More and more people don't have the ability to cook, many eat out, to the point where people are eating at fast food joints or restaurants daily or weekly.
Exactly. When a trend is happening simultaneously to millions of people, globally, it is more than just the individual at play here. People of course have individual responsibility, and if they put in the work they can overcome all of this. But it's not laziness at play here. It's broad, sociological factors that are affecting millions of people at the same time.
We could list hundreds of them if we wanted I'm sure.
1) Poor city planning limiting walking
2) Longer work hours with less time off
3) Rise of technology, television, internet, video games
4) Decreasing wages
5) Rising cost of food, housing, health insurance, everything
6) More sophisticated, intrusive, frequent, advertising designed using sophisticated psychological research
7) Changes in the content of food driven by corporate interests (decreasing cost of production/ mass production / shipping / storage, advancements in addiction/ craving creation in food products, higher dense caloric content).
8) Decrease in school funding for sports, recess, after school programs
9) Changes in media tone instilling fear about local communities leading to cultural shift with parents overprotecting their kids, letting the play outside independently less than in the past
10) Decreases in jobs requiring physical work, increases in jobs requiring long periods of sitting.
11) Cheap cost of mass producing unhealthy food, ability to keep it on shelves for long periods of time without deterioration
12) Rise of organic market inflating prices
13) Urban food deserts
14) Corporate consolidation of farming, death of the local farm
15) Broken homes, more single parents, less cooking, less culture of food education / cooking education
We can probably keep going indefinitely. But I think the key is to really look at the issue of obesity sociologically when we try to create policy changes. When you're talking personal motivation and individual cases, its okay to talk about motivation/laziness/personal responsibility.