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Varicose veins occur when the valves in the veins fail to function properly, leading to improper blood flow, particularly in the legs. The veins are responsible for returning deoxygenated blood back to the heart, and they have one-way valves to prevent backflow. When these valves weaken or become damaged, blood can pool in the veins instead of continuing to flow toward the heart. This pooling results in the veins becoming enlarged, twisted, and visible just beneath the skin, which is characteristic of varicose veins.

In contrast, thrombosis refers to the formation of a blood clot within a blood vessel, which can occur in various types of vessels, not specifically as a result of valve failure in veins. Atherosclerosis involves the buildup of plaque in arteries, leading to restricted blood flow, and it is unrelated to the function of venous valves. Hypertension, or high blood pressure, is a cardiovascular condition associated with increased pressure in the arteries and doesn't directly stem from venous valve function. Thus, the failing valves in veins specifically lead to the development of varicose veins.