Obesity typically results from over-eating (especially an unhealthy diet) and lack of enough exercise.
In our modern world with increasingly cheap, high calorie food (example, fast food — or junk food), prepared foods that are high in things like salt, sugars or fat, combined with our increasingly sedentary lifestyles, increasing urbanization and changing modes of transportation, it is no wonder that obesity has rapidly increased in the last few decades, around the world.