Consuming less calories is the behavioural change that will reduce obesity. The problem is that people only do this temporarily and then start eating at a surplus again, which makes them put the weight back on.

If someone quits chain-smoking they'll feel healthier, lose their cough, improve their aerobic capacity etc. If they go back to smoking they'll go back to feeling how they were. It's the same with obesity - doing short stints of dieting is like quitting smoking for a couple of months and then starting up again.