Tightening up our jaws is one of our involuntary reactions to stress - it may be a kind of protective device, tightening up the cranium to protect the brain in dangerous situations. (The masseter muscles can load up your jaw with anything up to 200lbs). It's not a good long term strategy though... along with tight neck and shoulders, it leads to strain and restriction in several key areas, the cranial base/atlas, vagus nerves, jugular veins, TMJ's...
