Varicose veins are caused by damaged or defective veins, typically in the legs.  

The defective valves cause blood to be routed down the veins instead of up toward the heart every time you stand up.

Pressure builds up in your veins as this continues to occur over time causing varicose veins in your legs. 

While nobody knows exactly what causes varicose veins, there are several conditions that place extra pressure on the veins in your legs making it more likely you get varicose veins.