Why is it easier to gain fat than to get rid of it?
There is a biochemical dynamic in the body that I rarely see mentioned-
There is no direct pathway to convert carbohydrates to fat. But when we eat a lot of food, a lot sugar, ultra processed foods etc, one of the carbohydrate metabolites is Acetyl-CoA. Acetyl-CoA is a significant "fuel" source for fat synthesis. In other words, the energy for fat to easily get formed is there.
It is like overcoming a small hump and then sliding down a steep hill. I am showing it in the following picture:
The carbohydrate biosynthesis provides the energy to form Acetyl-CoA and push it up the hump. When the body wants to form fat, it is now very easy because there is a lot of "fuel". I have shown this as the easy slide downhill. Another analogy is akin to having a lot of dry kindling wood in a forest. One little spark and a forest fire (fat formation) easily roars.
There is no reverse biochemical pathway. There is no way the fat, that has formed, has a way to get converted back to carbohydrate (or protein). So once the fat has formed, it is impossible to push it uphill and revert it back to carbohydrate. So now you are stuck with the fat.
This is the biochemical reason, within the body, that it is easier to gain fat than get rid of it.