system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If I eat the same thing for several meals in a row, why do I get sick of it?

Is it because our bodies function best with a variety of food, or have I just conditioned to eat different foods at each meal?
gpt
It could well be a combination of the two.  When you experience a stimulus, such as eating food, your body produces hormones in response.  When you do pretty important things that your body wants you to do, it produces 'feel good' hormones such as endorphins - things like eating and sex lead to endorphin release, and it's how your body motivates you to do things.

When the body repeatedly encounters a particular stimulus, such as a flavour, say, it begins to down-regulate it.  The same stimulus will (temporarily) have less effect on your body.  Think of background noise that might seem loud at first but soon gets blocked out.  That means that you're getting less endorphins if you eat the same thing over and over, and your body pushes you to eat something else for that sweet endorphin rush.