system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is it uncomfortable to exercise? It does not seem to make evolutionary sense.
gpt
You should keep in mind that for essentially every animal but present-day humans (and even for some of us today), famine is much more likely than feast. So all creatures are evolved with an emphasis on gathering, storing, and preserving reserves of energy in case of famine.

There is a part of your brain that, towards that end, wants you to never exert more energy than you need to. Why run when you can walk? Save the calories. Why walk when you can sit? Save the calories. Why do crunches when you could... not? Save the calories. 

Of course, for most of us today, we have too much food, and all could stand to burn extra calories. But our bodies don't know that yet. They still assume we're one bad season away from starving to death.