The reasons that stool withholding develops are complex. For some children it may start with a painful bowel movement. After that, they become fearful of having bowel movements and hold it in. No amount of reasoning can help a young child understand that if he holds it in, the stool will become harder and even more painful to pass. Young children only understand and deal with the fear and the pain of the moment.
Some children start withholding stool when toilet training begins.
For them there are not only issues of control, but also, fear of the unknown and comfort with the familiar. A child who has not experienced staying clean prefers his familiar dirty diaper to the scary toilet. And some children have great difficulty having a bowel movement on the toilet, especially if they are constipated. And so the vicious cycle begins. No matter what initially causes the child to withhold stool, the end result is the same.