system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If your body needs ~2000 calories a day to sustain itself, why do you get hungry during the day after eating 2000 calories worth of food in one sitting? 

This is probably a dumb question, but why do you need a certain amount of food to satisfy your hunger if your body doesn't actually need the calories? Why does your body tell you that its hungry if you already have your nutritional needs met?
gpt
The stomach isn't aware of how many calories it's consumed. It's not like there's a little meter in your intestines that flags the stomach to stop feeling hungry when the daily caloric requirement has been reached. 

The stomach simply makes you feel hungry when it's got nothing much to process. It wants to be continually doing *something*. So if you've got an abundance of low-to-moderate calorie foods, you just keep it partially full 24/7 and it hopefully averages out to around 2000 calories each day. Sometimes more (which causes your body to store the extra as fat), sometimes less (which causes your body to burn its fat stores to make energy). 

If you swallow a single pill that contains 2000 calories, then your daily caloric requirements are met, but your stomach is still saying "Hey dude, you've only given me one measly pill to work on all day, we're dying down here" and asks for more. 

This has become a big problem lately with the easy access to foods that pack a big caloric punch in a small physical package.