Nobody knows why stomach cancer develops. However, certain factors increase the risk:age(over 60); being male(men are twice as likely to get it); having helicobacter pylori infection(if it causes inflammation of the stomach lining, indigestion and stomach ulcers. Too much salt, red meat and foods that are smoked or that contain preservatives especially nitrosamines may increase the risk. Family history of stomach cancer, type A blood group, smoking and too much alcohol, atrophic gastritis(an illness causing the stomach lining to waste away), acid reflux, pernicious anaemia (where the body doesn't absorb enough vitamin B12 from the diet) or being overweight all increase your risk of getting stomach cancer.
Attribution: BUPA