Why do people let themselves get morbidly obese?

I don't understand how it even happens. I eat more than the average person, don't exercise much(I know, I need to more but it is winter)... and I am only slightly overweight. In fact, I've dropped 5 pounds in the past few weeks from little effort. My metabolism is on the slower end too. How do people get so obese?
Why do people let themselves get morbidly obese?
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