system
Given the following post, respond as if you're commenting on Reddit:
human
ELIF.Why does it feel so good to take a great big shit?

Besides the basic overall good feeling of no longer having to poop, why does it feel so fucking good to take a big dump?  The kind where you stretch it out because you have the place all to yourself.
gpt
This thread already has a pretty obviously awesome answer, but to throw something in the mix.

When you got poop up your butt, it is putting pressure on some special butt sensors.

Those butt sensors are sending signals to you to get that poop out of there so you can make room for more activities and such.

So they cause you intestinal distress. Sometimes in the form of pressure, or even in the form of your digestive system cramping up by undergoing rhythmic contractions to push the shit out.

All that stops it from succeeding is the semi penetrable barrier that is your anus. And you good sir are its master. 

As soon as your poops out, your butt sensors got no more signals, and you are now free of the constant waves of pressure.