If you employ a caloric restriction diet, you must do something to keep your metabolism from down-regulating, by whatever means that works for you. Otherwise, you will get stuck at a weight-loss plateau that is probably well above your goal weight, and further restricting calories from there might just worsen your situation.
You can't push on a string. Don't consume fewer calories than you burn; burn more calories than you consume. It's a fine hair to split, but be aware that they are not two independently controllable variables.
No you don't. You will always burn calories by simply existing. You can get no exercise and still lose weight simply by cutting calories. Cutting 100 calories from your daily diet may be matched one-to-one with metabolism decreases, but I guarantee that if you cut 1,000 calories from your daily diet you will lose weight.
Hint: nobody's goal is just weight loss.
See, your goal is not just to lose weight. Your goal is to lose fat. Your body will burn a hell of a lot of muscle if you just sit still and not eat.
And becuase you're burning more muscle than fat, you're even going to lose more weight. Hooray!
For added bonus, you can lay in bed all day so that you lose bone density as well. Even more weight loss even faster! So much better BMI. Much healthy and awesome.
Point is, nobody's goal is JUST to lose weight.
I think most people agree that should be avoided.
However, this has many moving parts and cheating (mostly by consuming more calories) is ridiculously easy. My guess would be that that’s the effect many people see when they don’t succeed. All of this is really hard and I was lucky to be in an extremely privileged position where I could devote lots of my attention to this task. Not many people are so lucky. Motivation obviously plays a crucial role and I absolutely do believe that it is central to successfully losing weight. The mechanics are simple and do indeed work like clockwork. But that doesn’t mean you can make them work like clockwork for yourself …
So, yeah, the body may be astonishingly mechanical when you execute everything perfectly, but motivation is the crucial second parts that allows you to execute everything correctly. And it may well be that to keep motivated it helps to be active or to eat certain thing. (I, for example, know that eating pasta with tomato sauce – formerly my absolute favourite food – will keep me hungry for longer. Meanwhile the same amount of calories from some salad with a vinaigrette and grilled chicken breast will keep me feeling full for longer.)