My neuro says it is due to a lot of stress. It feels like a knife poking on my Left temple. Sometimes, I have on the right side but the L hurts more frequently. I always had a ton of stress but I didn't have headaches.
Temple region headaches are typically from stress, but if they're severe, temporal arteritis should be considered. You may want to ask your neurologist about that possibility if it's severe, and an ESR blood test screens for it.