I think a lot of folks undertake exercise/diet programs that are fundamentally unsustainable for whatever reason. They can work like heck for a while but then burn out in a couple of months or a year. Very few of us can actualize our goals through sheer discipline, will power. This is why I think it is important to find a form of exercise that is genuinely pleasurable. If you hate riding bikes, find something else that is fun. If you hate your diet, adjust it so that it is more reasonable for you.
One of the primary problems is an Aristotelian one in that some folks grow up not finding any physical activity pleasurable at all. This is tough to deal with because the person has to overcome the notion that physical activity isn't pure suffering having had no prior experience with the pleasures of exercise.
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