1: The tooth can be close to the sinus. If the sinus is congested this pressure may lead to a toothache and the sensation of a headache.
2: When teeth hurt we tend to clench and/or grind more although we may not be aware that we do this. This can lead to stress of the muscles which can be felt as a headache as well.
3: The nerve innervation of the tooth is connected to the nerve innervation of the remainder of the face. This can lead to referred pain which can be felt as a headache as well.
1: Large decay into the nerve causing dental pain.
2: Current infection and/or dental pain due to crack into the nerve.
3: Traumatic lesion entering the nerve.
4: Prophylactic to allow for appropriate restoration.
If you have any concerns I would recommend talking to your dental professional with regards to how your treatment is specifically tailored to you.