Causes. Doctors do not know the exact cause of diverticulosis. Even the suspicion that a diet low in dietary fiber is uncertain. Still, a doctor who has a patient with diverticulosis will ask them about their diet and their overall lifestyle. Constipation may also be a factor if the person frequently strains at stool. This is why diverticula are most often found on the left side of the colon, which is where the most pressure is applied. Other causes may be a sluggish intestine or some problems with the gut microbiome.