Fatty liver disease is an increasingly common condition that currently affects a third of the population. The most common cause of the disease is obesity. Extra fat in your body from weight gain accumulates in your liver, causing it to swell. Eventually, the cells in your liver will be so overburdened that they die. New cells grow to replace them, but those cells also contain fat. As liver cells continue to die off and regrow rapidly, it causes scarring of the tissue surrounding the organ.