system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does stretching do and why does it feel so amazing?
gpt
when you sit still or sleep for a long time your muscles are not moving and blood has a hard time getting to all the cells and giving them more good food and energy and removing bad things. If you move your muscles around it helps to get blood to hard to reach places so that it can do its job better and get your nerves ready to start firing and used to working again! Which is good for your body! 

Our brain likes to reward us to do things that are good for our body so that we do it more often, so stretching feels good so that we do it more often.