Why do guys clench their jaw?

Anonymous
I have a lot of guy friends who clench their jaw but i don't understand why they do it...
Habit
Vote A
Anger
Vote B
He's Shy
Vote C
Awkward
Vote D
he's hurt/in pain
Vote E
Other (in the comments please)
Vote F
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GirlGuy
Why do guys clench their jaw?
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