system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why it would take you months to get in shape and 1-2 weeks to lose it.

it's really hard to build up some muscle and lose that fat around your belly , you stay on diet and work-out regularly for a long period of time. but once you stop ! it take much less time to lose what you've built.
gpt
We humans were not always living in conditions of plenty. There was a time, and some still live this way, when we didn't know where our next meal would come from, or when it would be. To conteract this, our bodies have adapted in several ways to survive the lean times, and maximize our ability to make it to the next meal. 

The two adaptations that are most relavent to this question is the storage of fat, and the limitation of muscle growth. Fat deposites are our bodies way of storing energy for later, when there is not enough food to survive on. Whenever your body finds itself with a surpluss of calories (like when you have that double quarter pounder with cheese) it converts some of those calories into fat, which is stored somewhere in your body, usualy under the skin where it also insulates you, to help regulate your body temp.

Muscle is expensive, in terms of calories. It costs lots of resources to build, and just as many to maintain. Muscle tissue also burns many more calories than most other tissue, even at rest. Your body will only build more muscle when it is exposed to conditions that strain the upper limits of your current muscles abilities (your work-outs). When you stop working out, your body no longer needs this expensive tissue and will divert resources away from it, allowing it to shrink, or sometimes even atrophe. This tearing down of muscle and build up of fat is much easier and, therefor, faster for your body to achieve.