system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it so difficult to gain muscle, but so easy to lose it?
gpt
Your body prefers to store extra energy as fat, because if you're ever running low on food your body can easily use fat for energy. To build muscle, you have to both "damage" the muscle by stressing it (e.g. lifting weights) and consume surplus calories that you can turn into more muscle tissue. Your body doesn't like to store extra energy as muscle, because it is more taxing (metabolically, ie. in terms of how much energy it takes for your body to maintain it) than storing that energy as fat. 

So, if you aren't actively working out, your body won't do anything to maintain the muscle and it will atrophy (ie. grow smaller/weaker). However, the extra muscle tissue you built when previously working out does not just disappear; you will see a "muscle memory" boost when you resume training because the muscle tissue you previously developed will "regrow" more quickly than it initially took when you started training. In other words, regaining lost muscle is much easier than growing new muscle in the first place.