As humans, we are highly adaptive. What this means is, we easily mould to the environment that we are in.
When it comes to eating, your body will do its very best to function optimally with what you give it, by adapting.
When you cut your calories, you are giving your body less energy than it requires to function.
So, it will do a number of things to combat this, the main being, reducing the amount of energy it uses (what it needs to function).
For example, if your body requires 2500 calories to function (otherwise known as your maintenance calories), if you eat 2200 calories, your body will (eventually) adapt to only require 2200 calories to function.
Simple really, just your body "slowing down" to function with what you give it. During this time you will lose weight.
To continue losing weight, one might reduce their calories further, to say, 2000 calories. Again, your body will (eventually) adapt to function with this amount of calories.
In summary, chronically cutting your calories will cause your body to slow down, and you will certainly feel it.
Low energy, brain fog, low sex drive, poor training performance... it's a long list.
Cutting calories is a temporary way to lose weight (specifically fat), not a long term way of eating.
Note: These example figures are not exact (nor is it quite that simple), they just aid the explanation.