What is it that makes me constantly clench my teeth/jaw?

Several reasons: Clenching your teeth can be referred too as grinding your teeth or bruxism. Clenching can be attributed to stress or malocclusion, certain medications and parkinson disease as well etc..... This can be corrected to a certain degree and you may need to wear an occlusal appliance. But you need to have a consultaiion with a TMJ specialist.

Stress: Or anger. Find ways to relax, or reduce your stress.

No one knows : There are may theories, but no one agrees on the cause.

Not clear: The cause of bruxism is not completely agreed upon, but daily stress may be the trigger in many people. It has been estimated that nearly 70% of bruxism occurs as a result of stress or anxiety that is affecting people subconsciously during sleep. Some people actually clench their teeth and never feel symptoms.