How Varicose Veins Happen
Usually varicose veins are caused by weak vein valves and walls.
In your veins are small one-way valves which open to let the blood flow through, and then close to avoid it coming backwards. Sometimes, the veins lose their elasticity, become stretched, which causes the valves to weaken.
When the valves do not function properly, the blood can start to leak and flow backwards. The blood collects in your veins, which causes them to sweal and enlarge.
We do not yet fully comprehend why the valves weaken and the veins stretch. In some cases, these conditions develop for no apparent reason.