...But to just say..."Gee, I am 100+ pounds overweight, but it is not my fault...this is my body's set weight..." is a cop out. I don't meant that to sound harsh. If you are comfortable there and love yourself at that weight, great! More power to you!

But for me, nope....I refuse to accept that I am "destined" to be heavy.
I don't think I was doing anything wrong when I was 100 pounds overweight. I ate about 10 servings of vegetables a day, almost never ate junk food, exercised regularly, drank lots of water (have to -- history of kidney stones) and stopped eating before I was full. I do realize that some people eat when they are stressed, but I do not. When I am anxious, I lose my appetite. If I am feeling down in the dumps, my husband has to urge me to eat, because I just don't feel like eating when I'm depressed. So, how was that extra 100 pounds my fault?
I have now managed to lose 30 pounds, and am "only" 70 pounds overweight, but I didn't do it by changing my lifestyle, because my lifestyle was fine to begin with. Basically, I did it by taking medication, which was the only way I could reduce the intense feelings of hunger and dramatic declines in body temperature (down to 94.5 degrees, at one point) that I get when I diet. The medication isn't instead of dieting; the medication makes it possible for me to diet.
I get very upset when people assume that obesity is always or almost always caused by poor behaviors. There is overwhelming evidence that many people are in fact destined to be fat. (I will be happy to give you references if you like.) Surgery does help some of these people, probably by changing the hormonal signals that their digestive tracts send to their hypothalmus, telling the brain how much they have eaten. But, surgery has its own risks, and currently there is no way to tell in advance who will benefit from it, so I decided not to try that.
...However, unless you have underlying medical condition, noone's "set weight" is 100+ pounds over the "standard." Our bodies are not built to get that large "naturally". Period.
Many people's bodies were built for an environment where people got 7 or more hours of exercise a day and almost never could find enough food to satisfy their hunger. Despite that, even when living a stone age lifestyle, some groups of people still weigh considerably more than what would be considered "normal" by the current medical establishment. For example, many of the Yanamamo, who live in the
Amazon jungle and get most of their food by hunting and gathering, are technically overweight, and some are obese. And, Pima Indians living traditional agricultural lifestyles in rural Mexico, where they spend 40 hours or more doing hard labor a week, still have high levels of obesity and diabetes. You put someone with those genes in a typical American environment, where food is always available and hardly anyone has 7 hours a day to exercise, and a lot of them are going to end up morbidly obese, no matter how hard they try to lose weight.
This varies tremdously by race. Western Europeans seem to have had the best access to food during past centuries, and they have about the lowest tendency towards Type 2 diabetes of any racial group, and some of the lowest tendencies of obesity (East Asians tend to be thin, but get Type 2 diabetes anyway.) Of course, given that the high-status group tends to be thin and that it is minority groups such as people of African decent and Native American decent (including many Latinos) that have the most tendency to gain weight easily, being fat becomes heavily stigmatized.
As for whether many obese people have an underlying medical condition, generally, anyone who looks closely at the physiology of very obese people finds that obese people have physiological factors that lead to weight gain. For example, when a normal person eats, their duodenum (the part of the intestines closest to the stomach) releases GLP-1, a hormone that tells the brain that food is being absorbed. In many obese people, little or no GLP-1 is released by the duodenum, and their brain can't tell that they have eaten until the person eats so much food that the duodenum can't absorb it all and leftover, unabsorbed food reaches the ileum (the end of the small intestine.)
In most cases, though, doctors don't bother looking for these physiological causes, partly because it's very expensive, and partly because until the past couple of years, there were no treatments for these problems anyway.
...But for me, nope....I refuse to accept that I am "destined" to be heavy.
I can't tell you if you are destined to be heavy or not. Set point seems to vary among people, not only in how high the set point is (i.e., 140 pounds versus 240 pounds), but in how strongly the body "defends" that setpoint (i.e., when a person loses weight, does their brain pull out all the stops to try to get it back, or not?) If you look at most people, their body temperature drops substantially when they diet -- it's one of the main ways that the body conserves fat. If you look at people who have lost a lot of weight and kept it off for years, though, their temperature doesn't drop when they diet -- their bodies don't seem to be trying to get back to their original weight. Maybe you are one of the few, the lucky few, who don't have a firm setpoint. For most people, though, when they try to lose weight, their body fights back to keep its fat, and it fights back
hard.