Well, you can't tar all fat people with the same brush - some are better marinated with BBQ sauce (JOKE!) - but as securespark states, there are genuine medical reasons that can contribute to some people being overweight.
On the other hand, as you suggest, there are many morbidly obese people who just don't have the willpower to lose weight and then try to blame it on genetics. (Again, some may be genuine, but most are not.)
I was shocked into losing almost 3½ stone a few years back when I realised that, at 17st 3lb, I was technically classed as obese and that most of my excess weight was around my middle - the worst place for future health problems. However, at 6' 2" tall, it wasn't really noticeable, and when I reached 13st 12lb, I looked like death warmed up, although I did feel a lot fitter and more energetic. Everyone said I looked sick even though, technically, I was still right on the borderline of healthy and overweight (BMI). I've gained almost 2st back (through choice), but have kept a healthy exercise program which has ensured that the weight has been gained all over, rather than just around my waist, and now I'm even fitter and feel healthier than I did after the weight loss.
Losing weight isn't easy, and keeping it off is even harder, but it can be done if you set yourself small goals at a time. I think some people look at their weight and think there's no way they can lose 5, 6... 10st, but if you think of losing 1st and then, when that's done, losing another, I believe most overweight people can achieve a decent amount weight loss.
