Varicose veins can occur in other areas but are commonest in the legs.
This is because of high pressure through them especially in the standing position. Blood that goes down to your legs through gravity must be pumped back up.
The veins in your legs have valves that only allow blood to go one way.
If you have valves that are damaged or weakened (As a result of genetics, pregnancy, obesity or trauma) blood then flows in both directions. This leads to pooling of blood in your lower legs as they are unable to be pumped up effectively.
This pooling makes the vein expand, ache and eventually varicose veins occur.
I mentioned lower legs, but this can also occur from the top down depending on how high the pressure is from the groin where it starts downwards.