When the protective enamel of the teeth is removed (from tongue piercings rubbing on them or from deliberate tooth filing), the exposed dentin is very prone to tooth decay from bacterial acids. Topical application of fluoride gels/rinses is strongly recommended. As well as decreasing the risk of infection, this should also decrease any tooth sensitivity.
While it is true that some primitive cultures used tooth filing to actually decrease their cavities because less plaque builds up on gapped teeth, this does not directly apply in modern times. This is mainly because these cultures were not exposed to refined sugars (and when they were, it markedly increased the occurrence of tooth decay) — our teeth have to survive in far more hostile environments.
The following is a note sent in by a BME reader:
"Once the outer layer of the tooth (enamel) is removed, one gets into the 'insulation' of the tooth. This insulation regulates the temperature of the nerve inside the tooth. Once the insulation is gone, as in the filing of a tooth, the nerve will slowly die. The nerve will get more sensitive first to hot and then to cold. After the sensitivity to cold it will get sensitive to pressure. And soon it will be so severe that even breathing will be more than excruciating. To reverse the pain and over all problems, one must get a root canal. The average price for a root canal is $250-500 per tooth. The endodontist (root canal specialist) will drill from the back of the tooth into the root canal itself. Then he will scrape the nerve and flush the now dead and infected nerve out. After about thirty minutes of extreme pleasure (the best feeling imaginable after having the nerve die and get infected), the endodontist will then pack the tooth and cap the hole off with a metal filling. You'll get pain pills (which are needed) and plenty of antibiotics resulting in abnormal gas production."