Yes. Some people are genetically predisposed to have thicker blood than normal and they are referred to as having a hypercoagulable state. Others have acquired hypercoagulability after certain surgical procedures or episodes of prolonged stasis such as the immobility of hospitalization or a long period of travel.
Yes. The body has an elaborate and exquisitely balanced system for making and dissolving clots. If this balance is thrown off, circumstances can arise where the body will have a tendency to either make clots more readily or be less able to inactivate the clotting “cascade”. Either way, clots are more prone to happen. Some people have inborn or acquired reasons for having a hypercoagulable disorder.