I think obesity is a very complex problem that people try to apply simple answers to. There are a lot of factors that weigh in on what causes it and I think it very much varies from person to person. I would never dare to apply the same reasons to why I am fat to someone else.
I know some of it is genetics, my Mom's side of the family is filled with short robust ladies who are the most wonderful women I have ever met and they embrace their largeness in a way I never could. But in my immediate family I am by far the largest. My older brother is 6'4 and weighs about 220, my little brother is 6'2 and weighs about 200, my baby sister is 5'1 and weighs about 110. So I don't think I can blame my problem soley on genetics. They all have very active jobs, kids, and have never really had to worry about their weight. My Mom has struggled with hers over the years but she's kept off about 100 lbs for the past five so I think she finally found a diet that works for her.
Some of it is emotional for some people. I know I had a terrible habit of stress eating, but the weight gain was more of a consequence of deeper emotional problems I was dealing with. I don't think I was binge eating because I was sad about being fat because I really didn't realize how big I had gotten until about a month ago - denial can be an amazing tool when you aren't ready to deal with something.
It seems like us ladies trying to lose weight are dealing with a double edge sword. We are punished and denied by the "skinnies" of the world for not being the ideal but then we are also shamed by fat acceptance people who think we should just accept and love who we are and not try to change at all. And both sides try to use really "one size fits all" arguements to explain why we are the way we are.
I know for me I want to be thinner because I'm going to die at a young age if I don't do something given my family history. With rampant diabetes, heart disease, and cancer running on both sides of my family I need to take responsibility for myself and do what I can to prevent what I can. I have no desire to be really really skinny - I would just like to be able to fit in a booth at a restaurant without having an anxiety attack.