Hmm, I'm 104kg/229lb roughly 6' and nearly 20; and at the moment I've a hell of a beer-gut (I became an alcoholic about a year and a half ago, now sorted that but it's darn difficult to shift the damage done).
I don't eat too healthily - my work at a convenience store means I leave home at about midday and don't get back until 10pm, so I'm eating right before I go to bed most times. I also don't get enough sleep (about 5 hours, not 8).
These are two major factors that contribute to my (and many other peoples' in this modern world) problems with being overweight.
However, I am not obese (just overweight with a beer gut) - the rest of me isn't fat, I try to go running a couple of times a week (when I manage to catch up on some sleep I'm going to make it a 5-times-a-week thing) and do regular weights (well, not regular enough because I'm too tired, but again improving it to 5x a week). Having toned muscles boosts your metabolism, helping you to loose weight without adjusting what you are eating too drastically (a little maths to ensure you aren't overloading on saturated fats or calories). I usually go for a 6-12 mile walk once or twice a week (I refuse to pay over £5 to get a bus for 6/7 miles), and my legs, shoulders and arms are solid muscle (played a lot of rugby in my youth).
I used a metabolic rate calculator, and based on my height it calculated that my body is using nearly 4000 calories a day to keep running. Since I only eat 2-3000 calories (my big issue is that with no time to eat I settle for very energy-rich foods like chocolate) - so I'm technically eating less than I'm theoretically burning. But I'm still not loosing any weight whatsoever (because of the times I eat).
Obesity itself can be a disease, as sources by previous posters. However, in a fair majority of cases it isn't, and is caused by lifestyle factors - either through shovelling food into your face, by eating at inappropriate times and not getting enough sleep. A side-effect of stress can be weight-gain too (notice these last 3 are all very difficult to avoid in the current working climate?).
I know full well in my case it's a problem with eating the wrong stuff, at the wrong times, not as much exercise as I'd like and so on. I know that I can't really adjust the times, and I like what I eat (since it's very sugar and fat-rich it has a very positive mood effect), so I'm trying to ramp up the exercise to try to shift the beer-gut. I'm not exactly lazy (I need to spend more time in bed lol), but certainly should be more active to improve my heart/lungs as well.