Let's use a made-up friend we will call "Stan" who is 35, 5' 8" tall, and 175lbs. At this time, he does not exercise but performs light, manual labor at his job and typical activities of daily life. Though he is a bit overweight, he maintains his weight by eating the amount of calories recommended for him and doing pretty much the same amount of physical activity each day. Stan doesn't want to exercise, but instead would like to lose 20lbs through dieting alone. Armed with the knowledge that 3500 calories equals one pound, he decides to drop 500 of the buggers from his daily required amount. At this rate, without changing his physical routines or quality of food, he anticipates losing the 20lbs in 140 days. Fair enough? Moving on.
Jump ahead 140 days and 20lbs lighter, Stan decides to try to maintain his new weight by going back to his original intake of calories. His activity levels are still the same and he's still consuming the right types and combinations of foods. Within a number of months, he can see the pounds slowly creeping back. He gets frustrated and sets up for another lap on the rollercoaster ride of weight loss. So, what did Stan do wrong?
Stan has two main problems which he has failed to address. The first is the most obvious. After he lost weight, his required caloric intake changed; it's now lower due to his new body composition. Since calories work both ways, in essence he is now overeating. A smaller body requires less energy to run (there's a lot more to this as someone who is naturally a certain size and someone who worked to get to that same size have different caloric needs, but that's for another time). The second problem is a bit more hidden but just as important.
During Stan's dieting, he didn't perform any more physical activity than normal. This is an issue because while his body was being deprived of calories it was compensating for that deficiency by taking the balance from nutrients already stored in his tissues and organs; he was losing muscle mass. This loss of muscle lowered his metabolic rate and therefore diminished the amount of calories his body was able to burn (we burn somewhere between 40 and 60 calories at rest each day for every pound of muscle). So now, not only is he consuming more calories than his new body needs, his new body is less efficient at creating caloric carnage than it was prior to the diet.
If Stan had started exercising while on his diet, his required caloric intake might have remained the same or risen higher than before, but his body would also be able to process the calories more efficiently and in greater numbers. From here, he'd have the option of maintaining the new Stan or continuing to improve his strength and physique.
In summary: If you are going to drop a few calories to lose some weight, make sure exercise is on the menu as well. Always remember to adjust the numbers when you see changes in your body. Finally, don't call it a "diet" because it's such a negative thing; nobody needs that when trying to stay on the positive side of fitness. Oh, and don't ask me where Stan works because I have no idea.