I do agree that some people are just genetically inclined to keep their weight. But a lot of of the time it is bad decisions too. My mother is considered medically obese and it really frustrates me. Not just because she is overweight, after all, she's my mother and I love her no matter what she looks like, but she and my stepfather live a sedate lifestyle, drink several liters coke and dr. pepper a week, snack on potato chips (him especially), add sour cream to everything, deep fry their own food when they do cook, etc. They also generally refuse to buy fresh produce because it is more expensive than prepackaged food. So, next to no fresh fruit&veg in their diets. They are basically the stereotype of an obese American. My mother has made some efforts to lose weight, and she has incorporated some frozen fruit smoothies and a few other healthy things into her diet, but still hasn't let go of the unhealthy things that I think cause her continued weight problems. I know her body is not "just built" this way because she gained the weight after some really rough stuff she went through 10+ years ago. She goes through periods where she will "play nice" and give up one of her vices (like the soda), lose a couple of pounds, and then go right back to it and gain the weight back. It's depressing for me to see, because I worry about her but there's nothing she will do and it's not something I can force on her. The other downside for me is she likes to make fun of my vegetarianism and a lot of my health choices, and I think when she does this she is subconsciously assuring herself that her diet is okay, somehow, like, that she's more "normal" than me. It's the only way I can think of to explain it.
Sorry, I know a lot of you could care less about my issues with my mom, but when the topic of obesity comes up, it's what always comes to the fore of my mind.