Heredity, or genetic predisposition to weak vein walls and vein valves, is the main reason why varicose and spider veins develop. This means that if you have developed a venous disorder, there is a 60-80% chance that someone in your family already has vein problems as well. There are also contributing factors that can accelerate the progression of the disease, including pregnancy, birth control pills, lack of exercise, prolonged sitting or standing, and even obesity. The aging process increases the likelihood of varicose and spider veins.