For diverticulitis to occur, there must be diverticulosis, the presence of diverticula. Diverticulosis can occur anywhere in the colon but it is most typical in the sigmoid colon, the S-shaped segment of the colon the left lower part of the abdomen. (Sides are from the patient's perspective so the left lower part of your abdomen is nearest your left hand).
The incidence of diverticulosis increases with age. Age causes a weakening of the walls of the colon and this weakening permits the formation of diverticula. By age 80, most people have diverticulosis.
A key factor promoting the formation of diverticulosis is elevated pressure within the colon. The pressure within the colon is raised when a person is constipated and has to push down to pass small, hard bits of stool ("rabbit droppings").
Diverticulosis sets the stage for inflammation and infection of the outpouching, that is for diverticulitis. (The "-itis" refers to inflammation.) It is potentially serious and can result in pain in the left lower abdomen, fever, nausea, vomiting, constipation and, paradoxically, diarrhea and frequent urination. Even graver consequences such as perforation of the colon and peritonitis are well known from diverticulitis.
The best way to avoid developing diverticulosis in the first place (aside from the impossibility of staying young) is by eating a proper healthy diet With plenty of fiber. A diet high in fiber keeps the bowels moving, keeps the pressure within the colon within normal limits, and slows or stops the formation of diverticula.