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Weight loss
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Vanity
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Health
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Illness
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Obesity
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Medical recommendation
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Lifestyle change
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Quality of life
It will affect how you approach the process of dieting and perhaps determine what your expectations are.
If your motive is purely health, and not simply weight loss, then it is easier to be patient.
If you are expecting to lose weight fast, or to meet a certain defined target: wedding, holiday etc... then dieting is harder because you are under pressure.