Why is it bad for you to hold bowel movements?

Stool retainer. Voluntary stool retention, when a repeated habit, may result in hard, dessicated stool that is painful & difficult to evacuate, promoting a vicious cycle of further stool retention to avoid pain. Other consequences: anal fissures, hemorrhoids, rectal prolapse, stercoral ulcer in the rectum, overflow incontinence around a stool plug, colonic distension with reduced tone, sphincter dysfunction, more.