Why does my tooth hurt when i scratch it?

Cavity or recession: receding gums will expose new,root surface that is not protected by enamel, this is a result of gum disease or zealous aggressive brushing. Abrasion and or cervical caries can cause the same thing, it is best to see a dentist to get to the bottom of his issue as the recession or abrasion will keep going and cause additional damage to the hole tooth and may be adjacent teeth as well.

Sensitive tooth: Cavity/abscess most common cause. Can also be from fracture, parafunctional habit, malocclusion, gum recession, among so many other possibilities. Bottom line, see your Den test and get it fixed before the symptoms become overwhelming at the most inconvenient time.