We're going to get into a bit of scat or to clue the poop box. :D
What's that barbaric word? You may find it strange, but it does exist. It's the fear of pooping. Yes, yes you read it right! The cause is often due to a traumatic shock in the past related to the "poop box". Anxiety, sometimes even a panic attack when it comes to having to go. It is the reptilian brain that triggers this.
There are different degrees of intensity, from simple fear to refusing to defecate because it's dirty. This can have serious consequences on health (constipation, obstruction...). However, this is how human beings are made. Everything that goes in must come out somewhere. We are no different from animals.
It is possible to get rid of this phobia whatever its origin or duration. All that is needed (easy to say but not always easy to do) is to identify the trigger. Self-therapy seems to be a good way to do this in order to release the emotional blocks.