The exact reason why cells within the stomach lining become cancerous is not fully understood. In many cases there is no apparent reason why someone has developed a stomach cancer. However, there are certain risk factors which do increase the chance of developing it. These include:
Age
It is more common in older people
Smoking
Stomach cancer is much more common in people who smoke
Diet
In some countries of the world (such as Japan) where people eat a lot of salt, pickled and smoked foods, there is a higher rate of stomach cancer. Eating a lot of fruit and green vegetables is known to reduce the risk of stomach cancer
Helicobacter Pylori
This is a very common, long term stomach infection in the United Kingdom, and the vast majority of people with this infection do not develop stomach cancer. However, it does very slightly increase the risk of causing stomach cancer.