One of the reasons getting regular colonoscopies is critical is because of the strange way our colon works. The vast majority of the diseases that can happen in large intestine occur in the mucosa (the most superficial layer), while the innervation lies in the muscularis propria (a deeper layer that provides peristaltic movement). For that reason, pain almost never correlates with ongoing pathogenesis. If you wait till you see even the tiniest symptom (constipation, diarrhea, miniscule amount of blood) before getting checked out, your survivability for a diagnosis of colon cancer drops below 50%.