DVT's can be caused by injury, genetic
conditions, and aging. In my case which is a genetic
condition the opposite of hemophilia (the blood doesn't clot) my blood clots easier, and more importantly doesn't stop clotting. This is called Factor V Leiden, and anyone with a clot should be tested for it. Sometimes it can be treated with an Aspirin a day, but once a clot forms, then there is a need to be on blood thinners, at least for a while.