One of the reasons I've always hated exercising is sweat.
When you're heavy, you're almost always sweating, even when sedentary. Combined with the body issues that come along with obesity, you begin to associate sweating with simply being fat, gross, ugly, etc. More than that, you begin to assume that when others see you sweating, they're actively judging you. So you spend an inordinate amount of time and energy avoiding that state. You drive instead of walk. You wear sandals instead of shoes (your feet are excellent heat sinks). You dodge potentially-outdoor social activities in favor of air-conditioned solitude. You actively avoid most/all forms of exercise.
These decisions cause a vicious cycle. They lead to paths of less and less activity, further ostracization (perceived or otherwise), and more self-esteem issues, which, if you're anything like me, can be temporarily forgotten with a good meal (and by "good" I mean terrible), causing further damage.
It all seems obvious, but when you're obese, you're often also in denial. And that's the core of it, really. Being sweaty is a reminder you can't ignore, and knowing others can't not notice (and judge) is terrifying.
My first long walks with Ruby up the Hill got me past that. Anyone trekking from downtown to 16th and Madison is going to break a sweat, fat or fit. That small thing - that kernel of acceptance that it's a natural (and vital) function which everyone experiences - has helped me re-position it in my mind. It also helped me alter my self-perception. I am what I am, and while that's okay, I can be better. It was getting over these proverbial "humps" that eventually lead to the decision to finally say "Fuck it" and start doing something.