It's half the basis of CBT lol. Changing habits first changes your mood down the road if you keep it up. It almost never works the other way around. You don't magically lose weight by thinking about eating less. You eat less, then the weight comes off.
Those of us for whom CBT didn't work*, are glad for those it did.
And if you've only ever been on the supply side, I'd say that I'd like to see your opinion once you've been through whatever it is that causes the need for it in the first place, but I wouldn't wish that on anyone.
* "Didn't work" is a gross oversimplification of course. Helpful techniques can be learned, but CBT is not the cure-all some people think it is.