Bob Greene discusses food addiction.

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Oprah's trainer and confidant Bob Greene, creator of The Best Life Diet, says every time Oprah falls off the weight loss wagon, it's harder for her to get back on. "One of the problems is her own success," he says. "She doesn't realize that the rules change. You know, age is a bear. You can't go back and work the same [as] when you were 30 or 40 when you're in your 50s."

Oprah's thyroid issue also made the process more difficult. "Does she use that as an excuse or did she? Absolutely," he says.

No matter who you are, Bob says a food addict can't conquer his or her addiction completely. "This is a problem in your life that's probably going to be a problem in your life for the duration of your life," he says. "Your results are simply about how you manage that problem."

Though he's never discussed it with Oprah, Bob shares a theory about his friend. "She didn't really learn how to be happy. I think she learned more survival tools and not how to be happy," he says. "That's where Oprah has a lot of work to do."

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