Because in this day and age we have a lot of misconceptions as far as health goes, so even people who think they're being healthy have a hard time losing weight because of what the food industry has told them their whole lives. Also, a huge percentage of us don't get as much exercise as we should because our modern lifestyles don't always call for it. The result is that a lot of people have the genetic predisposition to pack on the fat (which would have actually been a huge benefit for active pre-civ humans with good diets) and thus have a harder time losing it.
If we were all still living out in the woods and eating what we hunted or foraged, we would have no problem staying skinny. But these days a lot of people struggle with weight, and as such a lot of other people like to praise and encourage them for successfully losing it. Assuming that every fat person got that way because they're just a stupid fat slob and saying they don't deserve praise is ridiculous; there are a lot of people out there that just don't know much about what they should be eating, don't have a lot of money for healthy food, and don't have a lot of time to exercise. When they put forth the effort to get in shape a lot of people consider that an accomplishment.
Just being skinny already isn't impressive because there was no struggle involved. Lots of skinny people are skinny despite treating their bodies horribly. But a person who was heavy and then lost the weight through determination and discipline actually did something that took a lot of effort and a big change to their lifestyle.
Also, fat people losing weight just makes for good, uplifting stories for the other fat people to cling to. It's sort of like that whole spiel the one guy makes in The Hunger Games. The food industry still wants us fat and eating like crazy, but give us a little hope and it will keep us going.
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Bloodbath Paddy Wagon Ultra 9
