Why do people get blocked veins and arteries?

Family DNA. Arteries become blocked with arteriosclerosis, and it tends to run in the family, and diet/lifestyle contributes to the progression. Veins become blocked with blood clots, and heredity can make one more prone to development of blood clots. Venous clots occur in the deep and superficial veins, and the former type requires anticoagulation. Prolonged travel, surgery are risk factors for clots.