Why does tn pain come and go? why isn't it constant like most other pain?

Intermittent: Trigeminal neuralgia pain occurs when a branch of the trigeminal nerve becomes inflamed or over-excited. When this is relieved the pain resolves, but can be triggered again if the nerve is irritated.

Good question: Tn pain- trigeminal neuralgia- is not very well understood. The mechanism of pain, often short bursts of discomfort on one side of the face that come and go, are confusing. I would start by see a neurologist.