I think that's a question every overweight person has asked themselves. Sadly it's also one that non overweight people ask too. Even when they don't say it to you, you can sometimes see the question in their eyes. "how did you get so fat?" It feels like the implication is "Why didn't you stop yourself?"
It's not an easy question to answer. There are a lot of reasons for overeating. A lot of reasons to not exercise and it doesn't just happen overnight, but you don't always see it coming. First you gain a little and worry. If you're a stress eater that will trigger more eating. The numbers on the scale are stressful, so you don't use it anymore. After a little while you avoid looking down in the mirror, you only concentrate on your face. You start to avoid pictures and in some cases outings with friends and family. I have missed events that I would have liked to attend because I didn't want them to know how big I had gotten.
It's not like I was able to hide from my body. It was with me, but if I avoided looking at it, then I could avoid dealing with it - for awhile. Clothes in the large sizes help you hide from the truth too. Stretch knits, elastic waists. I still have some clothes that I can wear now - 150lbs away from my highest weight.
Avoiding situations where I was reminded about my size was also key. If I didn't enter the situation I wouldn't have to acknowledge that I didn't fit.
Then I started to lose weight. After people started noticing and telling me that I had lost at least 40lbs I decided to get on the scale. Imagine my shock when it read what I had guessed my starting weight to be. I was at least 80lbs heavier than I thought - probably more. When you get that big it sometimes takes a lot of loss for people to notice.
Avoidance is a powerful temptation. Today I still find myself slipping into those behaviors but I make a conscious effort to overcome them. I look at my whole body every day. Naked. I smile and pose for pictures and I take time to appreciate myself. I have a long way to go, but I am going to get there AND I am going to be satisfied with who I am along the way.