Note the normal looking hepatocytes to the right in this image - polygonal cells with a central vesicular nucleus and abundant pink cytoplasm. Towards the left are many cells that contain a large empty looking space. This is the fat that has been extracted during the histological processing.
Alcohol is the most common cause.
Toxic, metabolic and hypoxic conditions of all types. Liver is the principle organ of fat metabolism and synthesis. Interference with mobilizing triglycerides out of the liver is the usual mechanism. This happens when toxins or agents affect the mitochondrial and microsomal functions resulting in defective oxidation of fatty acids and aberrant mitochondrial function. Factors casuing fatty change do not all act in the same way.