"Why is it so hard to keep the weight off after I lose it?"  This is probably the most frequent question I have been asked over the past 17 years.  And, I have an answer.
Most people lose weight by going on a calorie restrictive diet.  They lose a few pounds, go off the diet and all of a sudden the weight comes back, along with a few extra pounds for good measure.  Sound familiar?
Diets do not work.  If they did we would all be thin.  All most diets do is screw up your metabolism by restricting calories.  
2 important things to remember:
1)  The body is an incredibly efficient machine.
2)  No amount of exercise can make up for a poor diet.

When you restrict your calories to 1200 or less per day your body's initial response will be to tap into fat stores for energy.  If you are someone who has been consuming a lot of calories (over 2500 per day) you will see a significant weight loss.
But your body is a machine and it is quite efficient.  Once it gets used to the lower number of calories, it will adjust your metabolism so that you don't burn as many calories per day.  How long does that take?  There is no magic number, everyone is different. For some it will happen is weeks, and for others it will happen in months. 
So, now your metabolism has shifted down to accomodate the 1200 calorie a day diet you've been on, but now you've gone back to consuming 2500+ calories!  The body's natural response will be to store all of those extra calories as fat.  That's why a lot of people wind up gaining back more than they lost.
But some of you will say, "But I started working out!  Shouldn't that make up for the extra calories?"  The answer is no.  Because the majority of the time people go back to poor eating habits.  And, as my clients have heard me say over and over, THERE IS NO AMOUNT OF EXERCISE THAT CAN MAKE UP FOR A POOR DIET!

The only way to lose weight and keep it off is through healthy lifestyle changes.  I started running to lose weight and never lost more than 10 lbs, even though I could run a half marathon!  I did not take off the weight and keep it off until I adopted healthy eating habits.  This was 33 years ago.  I still eat very healthy and exercise moderately.