Here's the "non-technical" person's view -- your body gets "comfortable" when it gets used to a particular eating pattern -- in the same way that it gets accustomed to a particular training regimen or cardio. When you hit a plateau, its time change what you are doing - sort of shock your system -- an example is when people follow a low carb diet for ever and can't understand why they arent' losing weight. Then toss in some more carbs and all of a sudden they are losing weight again, have more energy and off of MORE food than LESS.
People tend to lose more weight at the beginning of a diet because the body is responding to a big change in its situation, whereas after being on the diet for a while, it will adapt. That's why instead of sticking to the same diet, you need to constantly tweak it to get more efficient results as the body gets conditioned to the current demands.
The cool thing about the body is that it will adapt to most any situation you present it with, or at least try to. But when it gets adapted and you aren't at your goal yet, time to challenge it again to get it to readapt until you hit your goal. At that point, you want to maintain, but you'd probably still need to change up every now and then so it doesnt' get sluggish.