We've all been there.  Diet; workout; lose weight; gain it all back.  

Why?  

Here's why, according to Fox News

The wrong mindset:  We have not accepted that eating healthy is a life long commitment.  This is why fad diets never work, because you can't sustain that way of eating over the long term

Lack of sleep:  Fatigue is not your friend.  Fatigue makes us crave junk food, which is why experts say get 7 to 8 hours of sleep a night

Stress:  Here comes that word again... cortisol.  That's the hormone that helps us function under pressure, but when it kicks into overdrive, it also kicks up our cravings for fatty foods.  

Medical condition:  Thyroid disorder can mess with your weight, along with menopause, andropause, and andrenal fatigue.  Your physician can help you out here.