Just as your body has clotting factors that help form clots, there are also anti-clotting factors in your blood that prevent clots; these are called "anticoagulants." The purpose of anticoagulants is to stop clots from forming when your body doesn't need them.
But, since both of these types of factors are floating in your blood, how does your body know when a clot needs to be formed? Normally, there are more anticoagulants in your blood than activated clotting factors. And, unless a blood vessel is damaged, clots don't form.
But, if this balance is disturbed in either direction, it can be harmful. Too many anticoagulants make it hard for your body to form clots, even when they're really needed. And not enough anticoagulants may cause you to form unwanted clots.
Another thing that helps your body keep this balance is that blood is always flowing, which naturally mixes up the clotting and anti-clotting factors, and keeps them both from getting too concentrated in one place. When blood is NOT flowing smoothly, platelets and clotting factors can collect in pools, and become activated. This can cause a clot to form even when the blood vessel isn't damaged, such as inside the leg veins of an inactive person.
Certain health problems can also tip the balance toward making unwanted clots. We'll discuss these conditions a little later.