system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do muscles hurt the day after they are damaged, by over exercising or injury from a fall, and not at the time of the damage?

If I workout too much, why don't my muscles hurt that day? Why is it not until the next day, or some time after the event that leads to the pain, that the muscles start to ache?
gpt
When you work your muscles, you are causing small amounts of injury that will need to be repaired to be stronger. When an injury occurs in your body, your body releases endorphins which are natural painkillers (the source of runner's high).

In the short term, they allow you to ignore the pain coming from the injury. Why? Because the humans that immediately collapsed in pain from injuries are the ones that get caught by the large animal chasing them or the ones that can't move away from an impending landslide after getting hit with a rock. 

The natural painkiller allows us to ignore our pain so we can get ourselves out of danger first and worry about recovery second.