An extremely common issue in American adults, vein disease is generally caused by damaged or weak vein walls or valves. Tiny valves in your veins work against gravity, pushing blood back up to your heart. When these valves become weak or damaged, they become inefficient, and blood begins to pool in your veins instead of traveling to the heart. This increases the blood pressure in your veins, causing them to swell and enlarge. Genetics play a role in the development of vein conditions, as do old age, obesity, pregnancy and standing for long periods of time.