With me, the weight seems to be fairly genetic, all the women in my family are big. And I've never "really" been slim, except for last year in the height of my training when I got to 58kg, but I really had to starve myself to get there and exercise excessively. I also figure being slimmer will aid my running and make me less likely to gain injuries in the future.
The exercise for me, is because my lifelong goal is to join the Army, but injuries that occured just as the cost cutting/downsizing happened means that there's no spots for me for the foreseeable future. So, next year myself and my hubby (who's an Aussie) are moving to Australia so I can join up there, and because we'd have a better quality of life. It explains why I have a tendency to get a little carried away with the exercise and end up hurting myself as I've drilled into my head a "no pain, no gain" attitude, and that I have to keep going no matter what as I wouldn't be allowed to drop back in basic training.
But the weight, I've always struggled with it to be honest. I love food, and I love a good glass (or bottle) of wine and keeping up with my hubby and his mates with a pint or 3. That combined with that my family all find it really easy to put on weight seemed to be a recipe for disaster. And then when I injured myself, what did I do to make myself feel better about not being able to run? Yup, drink wine and eat cake/biscuits/anything I felt I deserved for being miserable!