I think Mary hit on one of the things people have the most trouble with: taking responsibility for themselves. "it isn't my fault it's the high fructose corn syrup", "it's not my fault, all we have is fast food around the place I work", "it's not my fault, diet food is too expensive". I'm sure you all have heard the excuses. (See Mike's Hey Fattie... thread)
I once got snippy enough with an "oh I couldn't possibly do that!" person that I retorted, "then you will stay fat forever" (not my kindest moment

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