A 2015 Gallup poll reports that half of Americans are trying to lose weight.  Of those that do lose weight, about 95% of them will regain it (often plus a few extra pounds).  As explained by Dr. Linda Bacon in her book Health At Every Size: The Surprising Truth About Your Weight (p. 26): 

• Your body likes to maintain a status quo and keep your weight relatively stable; this range of stable weight is called your "setpoint."
• Your body  strongly protects against dipping below your setpoint, though most people's bodies are relatively less aggressive at preventing them from rising above their setpoint.  In other words, weight gain is relatively easy at the same time that weight loss may not be possible.
• When you lose weight and threaten the system, your body may react by raising your setpoint, protecting against future threat.

The moral of the story — dieting or restricting to lose weight is actually causing your body to want to maintain a higher weight.  

While this might feel frustrating, discouraging, or defeating, it's actually brilliant.  Your body is protecting you against the threat of starvation by moving your natural weight further away from the starvation point.  This is why we think about food more if we are hungry, and become obsessed with food if we are under our setpoint weight.  It's your brain telling you, "Feed me for goodness sake!"