Genetics and age. Macular degeneration is a condition of the aging eye. Most people would get macular degeneration if they lived to be 150 years of age. The macula deteriorates with age and develops / deteriorates sooner in some families than others. Associated factors that can speed the development of macular degeneration are high blood pressure, high cholesterol, smoking, uv sunlight exposure.
Multifactorial. Macular degeneration describes a lot of conditions potentially. Some are single gene genetic disorders such as stargardt macular dystrophy. The most common cause is age-related macular degeneration. The condition has a strong genetic association (many genes involved). Other factors such as age, smoking, diet and other health factors contribute to its causation.
God knows. A lot of research being done now. Seems to be a problem with metabolizing the rods and cones which leads to drusen, which affects the barrier to the vascular layer. Many theories out there.