People who are seriously obese are that way for a broad range of reasons. Some of these reasons are medical, some genetic, and some psychological, but all of these are generally ignored in favor of "you're just lazy".
How do I know this? When I was eight, my thyroid stopped working. Despite a family history of thyroid problems, my doctor didn't even deign to have me tested. He said that hypothyroidism was "overdiagnosed," and for the next four years I was subjected to an onslaught of dietitians and other specialists who treated me like I just wasn't trying hard enough.
Understand, my mother was counting the grapes she put in my lunch. No healthy amount of dieting could have allowed me to lose weight; I was gaining it because my body didn't work properly. That didn't matter though. Even after I was diagnosed, I never got an apology from anyone involved. They felt they were right to treat me the way they did; after all, I was fat, so I was clearly doing something wrong.
Eventually, I got treatment, but by that point I was already obese. Losing weight, as most sane people know, is much harder than gaining it. Furthermore, losing a lot of weight is not just *a little* harder than losing ten or twenty pounds; the increase in difficulty is exponential. After a certain point, your metabolic set point changes. That's the weight your body is naturally inclined to stay at. Losing weight once that has happened requires more time, effort and energy than it does before it happens. Add to that the fact that people who are obese have more weight to lose in the first place - and thus need to maintain that energy and effort longer - and it becomes a very difficult proposition. It's not impossible; I've lost great amounts of weight under the correct circumstances, and what Jeramy has achieved is remarkable, but that's the exception not the rule.
Let me be very clear about something else: fat people don't all hate exercise, and those of us who do often have reasons that go far beyond "Moving is too much work; lets watch TV instead".
As an eight, nine or ten year old fat girl, exercise was my own personal hell. It usually happened at school where I was picked last for any sport I participated in, and was then bad at it because I was slower than other students and got tired more easily. I found that intensely humiliating, which made it kind of hard to have fun. That's assuming the other kids weren't making fun of me, although that happened less often than one might think because I was inclined to fight back.
Despite those limitations, there were sports and activities I liked. I loved dancing, and enjoyed playing volleyball. I also really liked swimming, however through much of high school I'd have cheerfully died before subjecting myself to the kind of ridicule wearing a swimsuit would have brought me. Since no one wants to dance with a fat girl, that was mostly out too, and I certainly wasn't making our high school volleyball team. People talk about fat kids not wanting to participate in sports, but if you're remotely honest you'll recognize that most sports teams won't accept fat kids in the first place, and if they do, the kids tend to be treated poorly at best by both other children and coaches.
Since college, my weight has fluctuated depending on the quality of care I could afford (I need regular blood tests to keep my T4 levels stable, which I often couldn't afford), level of depression (a side effect of both my illness and of dealing with life as a fat woman in our hugely prejudiced society) and my general well being. Recently, I was in bed for months with a combination of (then undiagnosed) sleep apnea, undermedicated hypothyroidism, and a newfound allergy to garlic and onions. I gained a lot of weight because I could barely get out of bed, never mind make wise dietary choices and exercise daily. It takes energy to do those things, and I had less than none. When I finally stopped feeling sick, I felt irrationally *guilty* about gaining weight, despite the fact that I couldn't do much about it, and furthermore people have treated me differently since.
Just for the record, being fat is a all around miserable experience. Most days, I wake up to large portions of my body hurting. Things are often too small for me, seats uncomfortable, clothing hard to find. It takes me longer to do anything physical, and I run out of energy more quickly than my peers. That's not the worst part, though; far from it.
The worst part of being fat is having people treat you like crap and feel superior and justified while doing so. It's doctors who don't listen to you when you say something's wrong because they think you're not worth bothering with because you're fat. It's employers and potential employers looking at you and not seeing an intelligent, talented, articulate woman, but that fat girl. It's having to work twice as hard as anyone else at making friends, and even then knowing you don't quite fit. It's seeing people who look like you casually mocked and degraded everywhere you turn.
Above all, it's knowing that, in our society, it's more ok to be anything else - an alcoholic, an asshole, an incompetent idiot - than to be what you are and had little choice in.
It's nice for all the thin people out there that they were not born with the illness I have, or a slow metabolism, or clinical depression, or any of the other conditions that can lead to obesity. It's truly great that they'll never have to live with this, but it's not a virtue on their part; mostly it's luck of the genetic draw. Oh, I understand that many of them go to the gym daily and work to stay a size 2 instead of ballooning up to a size 6, but even if they didn't, they'd probably never end up where I am. They are not genetically predisposed to it.
You may say "sure, but this is just you"; it's not. If you care to see it, I'll happily show you a study proving fat women are discriminated against by employers; given equal qualifications, the fat woman loses simply because she's fat. The study showing bias in the medical community is linked above. I can provide more corroboration if you need it.
Frankly, I am an exceptionally intelligent woman with enormous creative energy. I've used that energy and talent to make a place for myself despite my appearance. If you think I'm lazy, talk to the people at the LARP I help run, or to my boyfriend who's constantly at me to not overcommit. I am not just "that fat girl," but that's not good enough. Being fat is not a moral failing; it's a physical condition. We treat other physical disabilities (and believe me, it is disabling) with significantly more respect. We even manage to treat addictions like the diseases they are most of the time. We won't be a decent, moral society until we treat people who are overweight, people like me, fairly and respectfully.